X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fdocs%2Fusers_guide%2Fglasgow_exts.sgml;h=5c809257b901db00a3b87676ca0789d79bea12c7;hb=277cd58374ee122c3ae4dc7d99a10971facaf830;hp=c7d50be0cd3aa18b0dfc55a22888a8f3f4e5aac1;hpb=c7f8f1e62b555462f98c3f813440559116033a99;p=ghc-hetmet.git
diff --git a/ghc/docs/users_guide/glasgow_exts.sgml b/ghc/docs/users_guide/glasgow_exts.sgml
index c7d50be..5c80925 100644
--- a/ghc/docs/users_guide/glasgow_exts.sgml
+++ b/ghc/docs/users_guide/glasgow_exts.sgml
@@ -1,1258 +1,726 @@
-
-language, GHC
-extensions, GHC
+
+language, GHC
+extensions, GHC
As with all known Haskell systems, GHC implements some extensions to
-the language. To use them, you'll need to give a -fglasgow-exts
--fglasgow-exts option option.
-
+the language. To use them, you'll need to give a
+-fglasgow-exts option option.
+
-
+
Virtually all of the Glasgow extensions serve to give you access to
the underlying facilities with which we implement Haskell. Thus, you
can get at the Raw Iron, if you are willing to write some non-standard
-code at a more primitive level. You need not be ``stuck'' on
+code at a more primitive level. You need not be “stuck” on
performance because of the implementation costs of Haskell's
-``high-level'' features—you can always code ``under'' them. In an
-extreme case, you can write all your time-critical code in C, and then
-just glue it together with Haskell!
-
+“high-level” features—you can always code “under” them. In an extreme case, you can write all your time-critical code in C, and then just glue it together with Haskell!
+
-
+
Executive summary of our extensions:
-
-
-
-
-
-
-Unboxed types and primitive operations:
-
-
-You can get right down to the raw machine types and operations;
-included in this are ``primitive arrays'' (direct access to Big Wads
-of Bytes). Please see and following.
-
-
-
-
-
-Multi-parameter type classes:
-
-
-GHC's type system supports extended type classes with multiple
-parameters. Please see .
-
-
-
-
-
-Local universal quantification:
-
-
-GHC's type system supports explicit universal quantification in
-constructor fields and function arguments. This is useful for things
-like defining runST from the state-thread world. See .
-
-
-
-
-
-Extistentially quantification in data types:
-
-
-Some or all of the type variables in a datatype declaration may be
-existentially quantified. More details in .
-
-
-
-
-
-Scoped type variables:
-
-
-Scoped type variables enable the programmer to supply type signatures
-for some nested declarations, where this would not be legal in Haskell
-98. Details in .
-
-
-
-
-
-Calling out to C:
-
-
-Just what it sounds like. We provide lots of rope that you
-can dangle around your neck. Please see .
-
-
-
-
-
-Pragmas
-
-
-Pragmas are special instructions to the compiler placed in the source
-file. The pragmas GHC supports are described in .
-
-
-
-
-
-Rewrite rules:
-
-
-The programmer can specify rewrite rules as part of the source program
-(in a pragma). GHC applies these rewrite rules wherever it can.
-Details in .
-
-
-
-
-
-
-
+
+
+
+
+
+ Unboxed types and primitive operations:
+
+ You can get right down to the raw machine types and
+ operations; included in this are “primitive
+ arrays” (direct access to Big Wads of Bytes). Please
+ see and following.
+
+
+
+
+ Type system extensions:
+
+ GHC supports a large number of extensions to Haskell's
+ type system. Specifically:
+
+
+
+ Multi-parameter type classes:
+
+
+
+
+
+
+ Functional dependencies:
+
+
+
+
+
+
+ Implicit parameters:
+
+
+
+
+
+
+ Local universal quantification:
+
+
+
+
+
+
+ Extistentially quantification in data types:
+
+
+
+
+
+
+ Scoped type variables:
+
+ Scoped type variables enable the programmer to
+ supply type signatures for some nested declarations,
+ where this would not be legal in Haskell 98. Details in
+ .
+
+
+
+
+
+
+
+ Pattern guards
+
+ Instead of being a boolean expression, a guard is a list
+ of qualifiers, exactly as in a list comprehension. See .
+
+
+
+
+ Data types with no constructors
+
+ See .
+
+
+
+
+ Parallel list comprehensions
+
+ An extension to the list comprehension syntax to support
+ zipWith-like functionality. See .
+
+
+
+
+ Foreign calling:
+
+ Just what it sounds like. We provide
+ lots of rope that you can dangle around
+ your neck. Please see .
+
+
+
+
+ Pragmas
+
+ Pragmas are special instructions to the compiler placed
+ in the source file. The pragmas GHC supports are described in
+ .
+
+
+
+
+ Rewrite rules:
+
+ The programmer can specify rewrite rules as part of the
+ source program (in a pragma). GHC applies these rewrite rules
+ wherever it can. Details in .
+
+
+
+
+ Generic classes:
+
+ (Note: support for generic classes is currently broken
+ in GHC 5.02).
+
+ Generic class declarations allow you to define a class
+ whose methods say how to work over an arbitrary data type.
+ Then it's really easy to make any new type into an instance of
+ the class. This generalises the rather ad-hoc "deriving"
+ feature of Haskell 98. Details in .
+
+
+
+
+
Before you get too carried away working at the lowest level (e.g.,
-sloshing MutableByteArray#s around your program), you may wish to
-check if there are system libraries that provide a ``Haskellised
-veneer'' over the features you want. See .
-
-
-
-Unboxed types
-
-
-
-Unboxed types (Glasgow extension)
-
-
-
-These types correspond to the ``raw machine'' types you would use in
-C: Int# (long int), Double# (double), Addr# (void *), etc. The
-primitive operations (PrimOps) on these types are what you
-might expect; e.g., (+#) is addition on Int#s, and is the
-machine-addition that we all know and love—usually one instruction.
-
-
-
-There are some restrictions on the use of unboxed types, the main one
-being that you can't pass an unboxed value to a polymorphic function
-or store one in a polymorphic data type. This rules out things like
-[Int#] (ie. lists of unboxed integers). The reason for this
-restriction is that polymorphic arguments and constructor fields are
-assumed to be pointers: if an unboxed integer is stored in one of
-these, the garbage collector would attempt to follow it, leading to
-unpredictable space leaks. Or a seq operation on the polymorphic
-component may attempt to dereference the pointer, with disastrous
-results. Even worse, the unboxed value might be larger than a pointer
-(Double# for instance).
-
-
-
-Nevertheless, A numerically-intensive program using unboxed types can
-go a lot faster than its ``standard'' counterpart—we saw a
-threefold speedup on one example.
-
-
-
-Please see for the details of unboxed types and the
-operations on them.
-
-
-
-
-
-Primitive state-transformer monad
-
-
-
-state transformers (Glasgow extensions)
-ST monad (Glasgow extension)
-
-
-
+sloshing MutableByteArray#s around your
+program), you may wish to check if there are libraries that provide a
+“Haskellised veneer” over the features you want. See
+.
+
+
+
+ Language options
+
+ languageoption
+
+ optionslanguage
+
+ extensionsoptions controlling
+
+
+ These flags control what variation of the language are
+ permitted. Leaving out all of them gives you standard Haskell
+ 98.
+
+
+
+
+ :
+
+
+ This simultaneously enables all of the extensions to
+ Haskell 98 described in , except where otherwise
+ noted.
+
+
+
+
+ :
+
+
+ Switch off the Haskell 98 monomorphism restriction.
+ Independent of the
+ flag.
+
+
+
+
+
+
+
+
+
+
+
+ See . Only relevant
+ if you also use .
+
+
+
+
+
+
+
+ See . Only relevant if
+ you also use .
+
+
+
+
+
+
+
+ See . Independent of
+ .
+
+
+
+
+
+
+ -fno-implicit-prelude
+ option GHC normally imports
+ Prelude.hi files for you. If you'd
+ rather it didn't, then give it a
+ option. The idea
+ is that you can then import a Prelude of your own. (But
+ don't call it Prelude; the Haskell
+ module namespace is flat, and you must not conflict with
+ any Prelude module.)
+
+ Even though you have not imported the Prelude, all
+ the built-in syntax still refers to the built-in Haskell
+ Prelude types and values, as specified by the Haskell
+ Report. For example, the type [Int]
+ still means Prelude.[] Int; tuples
+ continue to refer to the standard Prelude tuples; the
+ translation for list comprehensions continues to use
+ Prelude.map etc.
+
+ With one group of exceptions! You may want to
+ define your own numeric class hierarchy. It completely
+ defeats that purpose if the literal "1" means
+ "Prelude.fromInteger 1", which is what
+ the Haskell Report specifies. So the
+ flag causes the
+ following pieces of built-in syntax to refer to whatever
+ is in scope, not the Prelude versions:
+
+
+
+ Integer and fractional literals mean
+ "fromInteger 1" and
+ "fromRational 3.2", not the
+ Prelude-qualified versions; both in expressions and in
+ patterns.
+
+
+
+ Negation (e.g. "- (f x)")
+ means "negate (f x)" (not
+ Prelude.negate).
+
+
+
+ In an n+k pattern, the standard Prelude
+ Ord class is still used for comparison,
+ but the necessary subtraction uses whatever
+ "(-)" is in scope (not
+ "Prelude.(-)").
+
+
+
+ Note: Negative literals, such as -3, are
+ specified by (a careful reading of) the Haskell Report as
+ meaning Prelude.negate (Prelude.fromInteger 3).
+ However, GHC deviates from this slightly, and treats them as meaning
+ fromInteger (-3). One particular effect of this
+ slightly-non-standard reading is that there is no difficulty with
+ the literal -2147483648 at type Int;
+ it means fromInteger (-2147483648). The strict interpretation
+ would be negate (fromInteger 2147483648),
+ and the call to fromInteger would overflow
+ (at type Int, remember).
+
+
+
+
+
+
+
+
+
+&primitives;
+
+
+Primitive state-transformer monad
+
+
+state transformers (Glasgow extensions)
+ST monad (Glasgow extension)
+
+
+
This monad underlies our implementation of arrays, mutable and
-immutable, and our implementation of I/O, including ``C calls''.
-
+immutable, and our implementation of I/O, including “C calls”.
+
-
-The ST library, which provides access to the ST monad, is a
-GHC/Hugs extension library and is described in the separate GHC/Hugs Extension Libraries document.
-
+
+The ST library, which provides access to the
+ST monad, is described in .
+
-
+
-
-Primitive arrays, mutable and otherwise
-
+
+Primitive arrays, mutable and otherwise
+
-
-primitive arrays (Glasgow extension)
-arrays, primitive (Glasgow extension)
-
+
+primitive arrays (Glasgow extension)
+arrays, primitive (Glasgow extension)
+
-
+
GHC knows about quite a few flavours of Large Swathes of Bytes.
-
+
-
+
First, GHC distinguishes between primitive arrays of (boxed) Haskell
-objects (type Array# obj) and primitive arrays of bytes (type
-ByteArray#).
-
+objects (type Array# obj) and primitive arrays of bytes (type
+ByteArray#).
+
-
+
Second, it distinguishes between…
-
+
-
-Immutable:
-
-
-Arrays that do not change (as with ``standard'' Haskell arrays); you
+
+Immutable:
+
+
+Arrays that do not change (as with “standard” Haskell arrays); you
can only read from them. Obviously, they do not need the care and
attention of the state-transformer monad.
-
-
-
-
-Mutable:
-
-
-Arrays that may be changed or ``mutated.'' All the operations on them
+
+
+
+
+Mutable:
+
+
+Arrays that may be changed or “mutated.” All the operations on them
live within the state-transformer monad and the updates happen
-in-place.
-
-
-
-
-``Static'' (in C land):
-
-
-A C routine may pass an Addr# pointer back into Haskell land. There
+in-place.
+
+
+
+
+“Static” (in C land):
+
+
+A C routine may pass an Addr# pointer back into Haskell land. There
are then primitive operations with which you may merrily grab values
-over in C land, by indexing off the ``static'' pointer.
-
-
-
-
-``Stable'' pointers:
-
-
+over in C land, by indexing off the “static” pointer.
+
+
+
+
+“Stable” pointers:
+
+
If, for some reason, you wish to hand a Haskell pointer (i.e.,
-not an unboxed value) to a C routine, you first make the
-pointer ``stable,'' so that the garbage collector won't forget that it
+not an unboxed value) to a C routine, you first make the
+pointer “stable,” so that the garbage collector won't forget that it
exists. That is, GHC provides a safe way to pass Haskell pointers to
C.
-
-
-
-Please see for more details.
-
-
-
-
-``Foreign objects'':
-
-
-A ``foreign object'' is a safe way to pass an external object (a
+
+
+
+Please see for more details.
+
+
+
+
+“Foreign objects”:
+
+
+A “foreign object” is a safe way to pass an external object (a
C-allocated pointer, say) to Haskell and have Haskell do the Right
Thing when it no longer references the object. So, for example, C
-could pass a large bitmap over to Haskell and say ``please free this
-memory when you're done with it.''
-
-
-
-Please see for more details.
-
-
-
-
-
-
-
-The libraries section gives more details on all these ``primitive
-array'' types and the operations on them, . Some of these extensions
-are also supported by Hugs, and the supporting libraries are described
-in the GHC/Hugs Extension Libraries
-document.
-
-
-
-
-
-Calling C directly from Haskell
-
-
-
-C calls (Glasgow extension)
-_ccall_ (Glasgow extension)
-_casm_ (Glasgow extension)
-
-
-
-GOOD ADVICE: Because this stuff is not Entirely Stable as far as names
-and things go, you would be well-advised to keep your C-callery
-corraled in a few modules, rather than sprinkled all over your code.
-It will then be quite easy to update later on.
-
-
-
-<Literal>_ccall_</Literal> and <Literal>_casm_</Literal>: an introduction
-
-
-
-The simplest way to use a simple C function
-
-
-
-
-
-double fooC( FILE *in, char c, int i, double d, unsigned int u )
-
-
-
-
-
-is to provide a Haskell wrapper:
-
-
-
-
-
-fooH :: Char -> Int -> Double -> Word -> IO Double
-fooH c i d w = _ccall_ fooC (``stdin''::Addr) c i d w
-
-
-
-
-
-The function fooH will unbox all of its arguments, call the C
-function fooC and box the corresponding arguments.
-
-
-
-One of the annoyances about _ccall_s is when the C types don't quite
-match the Haskell compiler's ideas. For this, the _casm_ variant
-may be just the ticket (NB: no chance of such code going
-through a native-code generator):
-
-
-
-
-
-import Addr
-import CString
-
-oldGetEnv name
- = _casm_ ``%r = getenv((char *) %0);'' name >>= \ litstring ->
- return (
- if (litstring == nullAddr) then
- Left ("Fail:oldGetEnv:"++name)
- else
- Right (unpackCString litstring)
- )
-
-
-
-
-
-The first literal-literal argument to a _casm_ is like a printf
-format: %r is replaced with the ``result,'' %0--%n-1 are
-replaced with the 1st--nth arguments. As you can see above, it is an
-easy way to do simple C casting. Everything said about _ccall_ goes
-for _casm_ as well.
-
-
-
-The use of _casm_ in your code does pose a problem to the compiler
-when it comes to generating an interface file for a freshly compiled
-module. Included in an interface file is the unfolding (if any) of a
-declaration. However, if a declaration's unfolding happens to contain
-a _casm_, its unfolding will not be emitted into the interface
-file even if it qualifies by all the other criteria. The reason why
-the compiler prevents this from happening is that unfolding _casm_s
-into an interface file unduly constrains how code that import your
-module have to be compiled. If an imported declaration is unfolded and
-it contains a _casm_, you now have to be using a compiler backend
-capable of dealing with it (i.e., the C compiler backend). If you are
-using the C compiler backend, the unfolded _casm_ may still cause you
-problems since the C code snippet it contains may mention CPP symbols
-that were in scope when compiling the original module are not when
-compiling the importing module.
-
-
-
-If you're willing to put up with the drawbacks of doing cross-module
-inlining of C code (GHC - A Better C Compiler :-), the option
--funfold-casms-in-hi-file will turn off the default behaviour.
--funfold-casms-in-hi-file option
-
-
-
-
-
-Literal-literals
-
-
-
-Literal-literals
-
-
-
-The literal-literal argument to _casm_ can be made use of separately
-from the _casm_ construct itself. Indeed, we've already used it:
-
-
-
-
-
-fooH :: Char -> Int -> Double -> Word -> IO Double
-fooH c i d w = _ccall_ fooC (``stdin''::Addr) c i d w
-
-
-
-
-
-The first argument that's passed to fooC is given as a literal-literal,
-that is, a literal chunk of C code that will be inserted into the generated
-.hc code at the right place.
-
-
-
-A literal-literal is restricted to having a type that's an instance of
-the CCallable class, see
-for more information.
-
-
-
-Notice that literal-literals are by their very nature unfriendly to
-native code generators, so exercise judgement about whether or not to
-make use of them in your code.
-
-
-
-
-
-Using function headers
-
-
-
-C calls, function headers
-
-
-
-When generating C (using the -fvia-C directive), one can assist the
-C compiler in detecting type errors by using the -#include directive
-to provide .h files containing function headers.
-
-
-
-For example,
-
-
-
-
-
-typedef unsigned long *StgForeignObj;
-typedef long StgInt;
-
-void initialiseEFS (StgInt size);
-StgInt terminateEFS (void);
-StgForeignObj emptyEFS(void);
-StgForeignObj updateEFS (StgForeignObj a, StgInt i, StgInt x);
-StgInt lookupEFS (StgForeignObj a, StgInt i);
-
-
-
-
-
-You can find appropriate definitions for StgInt, StgForeignObj,
-etc using gcc on your architecture by consulting
-ghc/includes/StgTypes.h. The following table summarises the
-relationship between Haskell types and C types.
-
-
-
-
-
-
-
-
-
-
-C type name
-Haskell Type
-
-
-
-
-StgChar
-Char#
-
-
-
-StgInt
-Int#
-
-
-
-StgWord
-Word#
-
-
-
-StgAddr
-Addr#
-
-
-
-StgFloat
-Float#
-
-
-
-StgDouble
-Double#
-
-
-
-StgArray
-Array#
-
-
-
-StgByteArray
-ByteArray#
-
-
-
-StgArray
-MutableArray#
-
-
-
-StgByteArray
-MutableByteArray#
-
-
-
-StgStablePtr
-StablePtr#
-
-
-
-StgForeignObj
-ForeignObj#
-
-
-
-
-
-
-
-
-Note that this approach is only essential for returning
-floats (or if sizeof(int) != sizeof(int *) on your
-architecture) but is a Good Thing for anyone who cares about writing
-solid code. You're crazy not to do it.
-
-
-
-
-
-Subverting automatic unboxing with ``stable pointers''
-
-
-
-stable pointers (Glasgow extension)
-
-
-
-The arguments of a _ccall_ are automatically unboxed before the
-call. There are two reasons why this is usually the Right Thing to
-do:
-
-
-
-
-
-
-
-
-C is a strict language: it would be excessively tedious to pass
-unevaluated arguments and require the C programmer to force their
-evaluation before using them.
-
-
-
-
-
-
- Boxed values are stored on the Haskell heap and may be moved
-within the heap if a garbage collection occurs—that is, pointers
-to boxed objects are not stable.
-
-
-
-
-
-
-
-
-It is possible to subvert the unboxing process by creating a ``stable
-pointer'' to a value and passing the stable pointer instead. For
-example, to pass/return an integer lazily to C functions storeC and
-fetchC, one might write:
-
-
-
-
-
-storeH :: Int -> IO ()
-storeH x = makeStablePtr x >>= \ stable_x ->
- _ccall_ storeC stable_x
-
-fetchH :: IO Int
-fetchH x = _ccall_ fetchC >>= \ stable_x ->
- deRefStablePtr stable_x >>= \ x ->
- freeStablePtr stable_x >>
- return x
-
-
-
-
-
-The garbage collector will refrain from throwing a stable pointer away
-until you explicitly call one of the following from C or Haskell.
-
-
-
-
-
-void freeStablePointer( StgStablePtr stablePtrToToss )
-freeStablePtr :: StablePtr a -> IO ()
-
-
-
-
-
-As with the use of free in C programs, GREAT CARE SHOULD BE
-EXERCISED to ensure these functions are called at the right time: too
-early and you get dangling references (and, if you're lucky, an error
-message from the runtime system); too late and you get space leaks.
-
-
-
-And to force evaluation of the argument within fooC, one would
-call one of the following C functions (according to type of argument).
-
-
-
-
-
-void performIO ( StgStablePtr stableIndex /* StablePtr s (IO ()) */ );
-StgInt enterInt ( StgStablePtr stableIndex /* StablePtr s Int */ );
-StgFloat enterFloat ( StgStablePtr stableIndex /* StablePtr s Float */ );
-
-
-
-
-
-performIO
-enterInt
-enterFloat
-
-
-
-Nota Bene: _ccall_GC__ccall_GC_ must be used if any of
-these functions are used.
-
-
-
-
-
-Foreign objects: pointing outside the Haskell heap
-
-
-
-foreign objects (Glasgow extension)
-
-
-
-There are two types that ghc programs can use to reference
-(heap-allocated) objects outside the Haskell world: Addr and
-ForeignObj.
-
-
-
-If you use Addr, it is up to you to the programmer to arrange
-allocation and deallocation of the objects.
-
-
-
-If you use ForeignObj, ghc's garbage collector will call upon the
-user-supplied finaliser function to free the object when the
-Haskell world no longer can access the object. (An object is
-associated with a finaliser function when the abstract
-Haskell type ForeignObj is created). The finaliser function is
-expressed in C, and is passed as argument the object:
-
-
-
-
-
-void foreignFinaliser ( StgForeignObj fo )
-
-
-
-
-
-when the Haskell world can no longer access the object. Since
-ForeignObjs only get released when a garbage collection occurs, we
-provide ways of triggering a garbage collection from within C and from
-within Haskell.
-
-
-
-
-
-void GarbageCollect()
-performGC :: IO ()
-
-
-
-
-
-More information on the programmers' interface to ForeignObj can be
-found in the library documentation.
-
-
-
-
-
-Avoiding monads
-
-
-
-C calls to `pure C'
-unsafePerformIO
-
-
-
-The _ccall_ construct is part of the IO monad because 9 out of 10
-uses will be to call imperative functions with side effects such as
-printf. Use of the monad ensures that these operations happen in a
-predictable order in spite of laziness and compiler optimisations.
-
-
-
-To avoid having to be in the monad to call a C function, it is
-possible to use unsafePerformIO, which is available from the
-IOExts module. There are three situations where one might like to
-call a C function from outside the IO world:
-
-
-
-
-
-
-
-
-Calling a function with no side-effects:
-
-
-atan2d :: Double -> Double -> Double
-atan2d y x = unsafePerformIO (_ccall_ atan2d y x)
-
-sincosd :: Double -> (Double, Double)
-sincosd x = unsafePerformIO $ do
- da <- newDoubleArray (0, 1)
- _casm_ ``sincosd( %0, &((double *)%1[0]), &((double *)%1[1]) );'' x da
- s <- readDoubleArray da 0
- c <- readDoubleArray da 1
- return (s, c)
-
-
-
-
-
-
-
-
- Calling a set of functions which have side-effects but which can
-be used in a purely functional manner.
-
-For example, an imperative implementation of a purely functional
-lookup-table might be accessed using the following functions.
-
-
-
-empty :: EFS x
-update :: EFS x -> Int -> x -> EFS x
-lookup :: EFS a -> Int -> a
-
-empty = unsafePerformIO (_ccall_ emptyEFS)
-
-update a i x = unsafePerformIO $
- makeStablePtr x >>= \ stable_x ->
- _ccall_ updateEFS a i stable_x
-
-lookup a i = unsafePerformIO $
- _ccall_ lookupEFS a i >>= \ stable_x ->
- deRefStablePtr stable_x
-
-
-
-You will almost always want to use ForeignObjs with this.
-
-
-
-
-
-
- Calling a side-effecting function even though the results will
-be unpredictable. For example the trace function is defined by:
-
-
-
-trace :: String -> a -> a
-trace string expr
- = unsafePerformIO (
- ((_ccall_ PreTraceHook sTDERR{-msg-}):: IO ()) >>
- fputs sTDERR string >>
- ((_ccall_ PostTraceHook sTDERR{-msg-}):: IO ()) >>
- return expr )
- where
- sTDERR = (``stderr'' :: Addr)
-
-
-
-(This kind of use is not highly recommended—it is only really
-useful in debugging code.)
-
-
-
-
-
-
-
-
-
-
-C-calling ``gotchas'' checklist
-
-
-
-C call dangers
-CCallable
-CReturnable
-
-
-
-And some advice, too.
-
-
-
-
-
-
-
-
- For modules that use _ccall_s, etc., compile with
--fvia-C.-fvia-C option You don't have to, but you should.
-
-Also, use the -#include "prototypes.h" flag (hack) to inform the C
-compiler of the fully-prototyped types of all the C functions you
-call. ( says more about this…)
-
-This scheme is the only way that you will get any
-typechecking of your _ccall_s. (It shouldn't be that way, but…).
-GHC will pass the flag -Wimplicit to gcc so that you'll get warnings
-if any _ccall_ed functions have no prototypes.
-
-
-
-
-
-
-Try to avoid _ccall_s to C functions that take float
-arguments or return float results. Reason: if you do, you will
-become entangled in (ANSI?) C's rules for when arguments/results are
-promoted to doubles. It's a nightmare and just not worth it.
-Use doubles if possible.
-
-If you do use floats, check and re-check that the right thing is
-happening. Perhaps compile with -keep-hc-file-too and look at
-the intermediate C (.hc file).
-
-
-
-
-
-
- The compiler uses two non-standard type-classes when
-type-checking the arguments and results of _ccall_: the arguments
-(respectively result) of _ccall_ must be instances of the class
-CCallable (respectively CReturnable). Both classes may be
-imported from the module CCall, but this should only be
-necessary if you want to define a new instance. (Neither class
-defines any methods—their only function is to keep the
-type-checker happy.)
-
-The type checker must be able to figure out just which of the
-C-callable/returnable types is being used. If it can't, you have to
-add type signatures. For example,
-
-
-
-f x = _ccall_ foo x
-
-
-
-is not good enough, because the compiler can't work out what type x
-is, nor what type the _ccall_ returns. You have to write, say:
-
-
-
-f :: Int -> IO Double
-f x = _ccall_ foo x
-
-
-
-This table summarises the standard instances of these classes.
-
-
-
-
-
-
-
-
-
-Type
-CCallable
-CReturnable
-Which is probably…
-
-
-
-Char
- Yes
- Yes
-unsigned char
-
-
-
-Int
- Yes
- Yes
-long int
-
-
-
-Word
- Yes
- Yes
-unsigned long int
-
-
-
-Addr
- Yes
- Yes
-void *
-
-
-
-Float
- Yes
- Yes
-float
-
-
-
-Double
- Yes
- Yes
-double
-
-
-
-()
- No
- Yes
-void
-
-
-
-[Char]
- Yes
- No
-char * (null-terminated)
-
-
-
-Array
- Yes
- No
-unsigned long *
-
-
-
-ByteArray
- Yes
- No
-unsigned long *
-
-
-
-MutableArray
- Yes
- No
-unsigned long *
-
-
-
-MutableByteArray
- Yes
- No
-unsigned long *
-
-
-
-State
- Yes
- Yes
- nothing!
-
-
-
-StablePtr
- Yes
- Yes
-unsigned long *
-
-
-
-ForeignObjs
- Yes
- Yes
- see later
-
-
-
-
-
-
-
-Actually, the Word type is defined as being the same size as a
-pointer on the target architecture, which is probably
-unsigned long int.
-
-The brave and careful programmer can add their own instances of these
-classes for the following types:
-
-
-
-
-
-
-A boxed-primitive type may be made an instance of both
-CCallable and CReturnable.
-
-A boxed primitive type is any data type with a
-single unary constructor with a single primitive argument. For
-example, the following are all boxed primitive types:
-
-
-
-Int
-Double
-data XDisplay = XDisplay Addr#
-data EFS a = EFS# ForeignObj#
-
-
-
-
-
-instance CCallable (EFS a)
-instance CReturnable (EFS a)
-
-
-
-
-
-
-
-
- Any datatype with a single nullary constructor may be made an
-instance of CReturnable. For example:
-
-
-
-data MyVoid = MyVoid
-instance CReturnable MyVoid
-
-
-
-
-
-
-
-
- As at version 2.09, String (i.e., [Char]) is still
-not a CReturnable type.
-
-Also, the now-builtin type PackedString is neither
-CCallable nor CReturnable. (But there are functions in
-the PackedString interface to let you get at the necessary bits…)
-
-
-
-
-
-
-
-
-
-
-
- The code-generator will complain if you attempt to use %r in
-a _casm_ whose result type is IO (); or if you don't use %r
-precisely once for any other result type. These messages are
-supposed to be helpful and catch bugs—please tell us if they wreck
-your life.
-
-
-
-
-
-
- If you call out to C code which may trigger the Haskell garbage
-collector or create new threads (examples of this later…), then you
-must use the _ccall_GC__ccall_GC_ primitive or
-_casm_GC__casm_GC_ primitive variant of C-calls. (This
-does not work with the native code generator - use \fvia-C.) This
-stuff is hairy with a capital H!
-
-
-
-
-
-
-
-
-
-
-
-
-Multi-parameter type classes
-
-
-
-This section documents GHC's implementation of multi-paramter type
+could pass a large bitmap over to Haskell and say “please free this
+memory when you're done with it.”
+
+
+
+Please see for more details.
+
+
+
+
+
+
+
+The libraries documentatation gives more details on all these
+“primitive array” types and the operations on them.
+
+
+
+
+
+
+Data types with no constructors
+
+With the flag, GHC lets you declare
+a data type with no constructors. For example:
+
+ data S -- S :: *
+ data T a -- T :: * -> *
+
+Syntactically, the declaration lacks the "= constrs" part. The
+type can be parameterised, but only over ordinary types, of kind *; since
+Haskell does not have kind signatures, you cannot parameterise over higher-kinded
+types.
+
+Such data types have only one value, namely bottom.
+Nevertheless, they can be useful when defining "phantom types".
+
+
+
+Pattern guards
+
+
+Pattern guards (Glasgow extension)
+The discussion that follows is an abbreviated version of Simon Peyton Jones's original proposal. (Note that the proposal was written before pattern guards were implemented, so refers to them as unimplemented.)
+
+
+
+Suppose we have an abstract data type of finite maps, with a
+lookup operation:
+
+
+lookup :: FiniteMap -> Int -> Maybe Int
+
+
+The lookup returns Nothing if the supplied key is not in the domain of the mapping, and (Just v) otherwise,
+where v is the value that the key maps to. Now consider the following definition:
+
+
+
+clunky env var1 var2 | ok1 && ok2 = val1 + val2
+| otherwise = var1 + var2
+where
+ m1 = lookup env var1
+ m2 = lookup env var2
+ ok1 = maybeToBool m1
+ ok2 = maybeToBool m2
+ val1 = expectJust m1
+ val2 = expectJust m2
+
+
+
+The auxiliary functions are
+
+
+
+maybeToBool :: Maybe a -> Bool
+maybeToBool (Just x) = True
+maybeToBool Nothing = False
+
+expectJust :: Maybe a -> a
+expectJust (Just x) = x
+expectJust Nothing = error "Unexpected Nothing"
+
+
+
+What is clunky doing? The guard ok1 &&
+ok2 checks that both lookups succeed, using
+maybeToBool to convert the Maybe
+types to booleans. The (lazily evaluated) expectJust
+calls extract the values from the results of the lookups, and binds the
+returned values to val1 and val2
+respectively. If either lookup fails, then clunky takes the
+otherwise case and returns the sum of its arguments.
+
+
+
+This is certainly legal Haskell, but it is a tremendously verbose and
+un-obvious way to achieve the desired effect. Arguably, a more direct way
+to write clunky would be to use case expressions:
+
+
+
+clunky env var1 var1 = case lookup env var1 of
+ Nothing -> fail
+ Just val1 -> case lookup env var2 of
+ Nothing -> fail
+ Just val2 -> val1 + val2
+where
+ fail = val1 + val2
+
+
+
+This is a bit shorter, but hardly better. Of course, we can rewrite any set
+of pattern-matching, guarded equations as case expressions; that is
+precisely what the compiler does when compiling equations! The reason that
+Haskell provides guarded equations is because they allow us to write down
+the cases we want to consider, one at a time, independently of each other.
+This structure is hidden in the case version. Two of the right-hand sides
+are really the same (fail), and the whole expression
+tends to become more and more indented.
+
+
+
+Here is how I would write clunky:
+
+
+
+clunky env var1 var1
+ | Just val1 <- lookup env var1
+ , Just val2 <- lookup env var2
+ = val1 + val2
+...other equations for clunky...
+
+
+
+The semantics should be clear enough. The qualifers are matched in order.
+For a <- qualifier, which I call a pattern guard, the
+right hand side is evaluated and matched against the pattern on the left.
+If the match fails then the whole guard fails and the next equation is
+tried. If it succeeds, then the appropriate binding takes place, and the
+next qualifier is matched, in the augmented environment. Unlike list
+comprehensions, however, the type of the expression to the right of the
+<- is the same as the type of the pattern to its
+left. The bindings introduced by pattern guards scope over all the
+remaining guard qualifiers, and over the right hand side of the equation.
+
+
+
+Just as with list comprehensions, boolean expressions can be freely mixed
+with among the pattern guards. For example:
+
+
+
+f x | [y] <- x
+ , y > 3
+ , Just z <- h y
+ = ...
+
+
+
+Haskell's current guards therefore emerge as a special case, in which the
+qualifier list has just one element, a boolean expression.
+
+
+
+
+ Parallel List Comprehensions
+ list comprehensionsparallel
+
+ parallel list comprehensions
+
+
+ Parallel list comprehensions are a natural extension to list
+ comprehensions. List comprehensions can be thought of as a nice
+ syntax for writing maps and filters. Parallel comprehensions
+ extend this to include the zipWith family.
+
+ A parallel list comprehension has multiple independent
+ branches of qualifier lists, each separated by a `|' symbol. For
+ example, the following zips together two lists:
+
+
+ [ (x, y) | x <- xs | y <- ys ]
+
+
+ The behavior of parallel list comprehensions follows that of
+ zip, in that the resulting list will have the same length as the
+ shortest branch.
+
+ We can define parallel list comprehensions by translation to
+ regular comprehensions. Here's the basic idea:
+
+ Given a parallel comprehension of the form:
+
+
+ [ e | p1 <- e11, p2 <- e12, ...
+ | q1 <- e21, q2 <- e22, ...
+ ...
+ ]
+
+
+ This will be translated to:
+
+
+ [ e | ((p1,p2), (q1,q2), ...) <- zipN [(p1,p2) | p1 <- e11, p2 <- e12, ...]
+ [(q1,q2) | q1 <- e21, q2 <- e22, ...]
+ ...
+ ]
+
+
+ where `zipN' is the appropriate zip for the given number of
+ branches.
+
+
+
+
+Multi-parameter type classes
+
+
+
+This section documents GHC's implementation of multi-parameter type
classes. There's lots of background in the paper Type classes: exploring the design space (Simon Peyton
-Jones, Mark Jones, Erik Meijer).
-
+URL="http://research.microsoft.com/~simonpj/multi.ps.gz" >Type
+classes: exploring the design space (Simon Peyton Jones, Mark
+Jones, Erik Meijer).
+
-
+
I'd like to thank people who reported shorcomings in the GHC 3.02
implementation. Our default decisions were all conservative ones, and
the experience of these heroic pioneers has given useful concrete
examples to support several generalisations. (These appear below as
design choices not implemented in 3.02.)
-
+
-
+
I've discussed these notes with Mark Jones, and I believe that Hugs
will migrate towards the same design choices as I outline here.
Thanks to him, and to many others who have offered very useful
feedback.
-
+
-
-Types
+
+Types
-
+
There are the following restrictions on the form of a qualified
type:
-
+
-
+
-
- forall tv1..tvn (c1, ...,cn) => type
-
+
+ forall tv1..tvn (c1, ...,cn) => type
+
-
+
-
+
(Here, I write the "foralls" explicitly, although the Haskell source
language omits them; in Haskell 1.4, all the free type variables of an
explicit source-language type signature are universally quantified,
except for the class type variables in a class declaration. However,
-in GHC, you can give the foralls if you want. See ).
-
+in GHC, you can give the foralls if you want. See ).
+
-
+
-
+
-
- Each universally quantified type variable
-tvi must be mentioned (i.e. appear free) in type.
+
+ Each universally quantified type variable
+tvi must be mentioned (i.e. appear free) in type.
The reason for this is that a value with a type that does not obey
this restriction could not be used without introducing
ambiguity. Here, for example, is an illegal type:
-
- forall a. Eq a => Int
-
+
+ forall a. Eq a => Int
+
-When a value with this type was used, the constraint Eq tv
-would be introduced where tv is a fresh type variable, and
+When a value with this type was used, the constraint Eq tv
+would be introduced where tv is a fresh type variable, and
(in the dictionary-translation implementation) the value would be
-applied to a dictionary for Eq tv. The difficulty is that we
-can never know which instance of Eq to use because we never
-get any more information about tv.
+applied to a dictionary for Eq tv. The difficulty is that we
+can never know which instance of Eq to use because we never
+get any more information about tv.
-
-
-
+
+
+
-
- Every constraint ci must mention at least one of the
-universally quantified type variables tvi.
+
+ Every constraint ci must mention at least one of the
+universally quantified type variables tvi.
-For example, this type is OK because C a b mentions the
-universally quantified type variable b:
+For example, this type is OK because C a b mentions the
+universally quantified type variable b:
-
- forall a. C a b => burble
-
+
+ forall a. C a b => burble
+
-The next type is illegal because the constraint Eq b does not
-mention a:
+The next type is illegal because the constraint Eq b does not
+mention a:
-
- forall a. Eq b => burble
-
+
+ forall a. Eq b => burble
+
The reason for this restriction is milder than the other one. The
@@ -1262,312 +730,312 @@ out). Furthermore, floating them out increases sharing. Lastly,
excluding them is a conservative choice; it leaves a patch of
territory free in case we need it later.
-
-
+
+
-
+
-
+
These restrictions apply to all types, whether declared in a type signature
or inferred.
-
+
-
-Unlike Haskell 1.4, constraints in types do not have to be of
-the form (class type-variables). Thus, these type signatures
+
+Unlike Haskell 1.4, constraints in types do not have to be of
+the form (class type-variables). Thus, these type signatures
are perfectly OK
-
+
-
+
-
- f :: Eq (m a) => [m a] -> [m a]
- g :: Eq [a] => ...
-
+
+ f :: Eq (m a) => [m a] -> [m a]
+ g :: Eq [a] => ...
+
-
+
-
+
This choice recovers principal types, a property that Haskell 1.4 does not have.
-
+
-
+
-
-Class declarations
+
+Class declarations
-
+
-
+
-
- Multi-parameter type classes are permitted. For example:
+
+ Multi-parameter type classes are permitted. For example:
-
+
class Collection c a where
- union :: c a -> c a -> c a
+ union :: c a -> c a -> c a
...etc.
-
+
-
-
-
+
+
+
-
- The class hierarchy must be acyclic. However, the definition
+
+ The class hierarchy must be acyclic. However, the definition
of "acyclic" involves only the superclass relationships. For example,
this is OK:
-
+
class C a where {
- op :: D b => a -> b -> b
+ op :: D b => a -> b -> b
}
- class C a => D a where { ... }
-
+ class C a => D a where { ... }
+
-Here, C is a superclass of D, but it's OK for a
-class operation op of C to mention D. (It
-would not be OK for D to be a superclass of C.)
+Here, C is a superclass of D, but it's OK for a
+class operation op of C to mention D. (It
+would not be OK for D to be a superclass of C.)
-
-
-
+
+
+
-
- There are no restrictions on the context in a class declaration
+
+ There are no restrictions on the context in a class declaration
(which introduces superclasses), except that the class hierarchy must
-be acyclic. So these class declarations are OK:
+be acyclic. So these class declarations are OK:
-
- class Functor (m k) => FiniteMap m k where
+
+ class Functor (m k) => FiniteMap m k where
...
- class (Monad m, Monad (t m)) => Transform t m where
- lift :: m a -> (t m) a
-
+ class (Monad m, Monad (t m)) => Transform t m where
+ lift :: m a -> (t m) a
+
-
-
-
+
+
+
-
- In the signature of a class operation, every constraint
+
+ In the signature of a class operation, every constraint
must mention at least one type variable that is not a class type
-variable.
+variable.
Thus:
-
+
class Collection c a where
- mapC :: Collection c b => (a->b) -> c a -> c b
-
+ mapC :: Collection c b => (a->b) -> c a -> c b
+
-is OK because the constraint (Collection a b) mentions
-b, even though it also mentions the class variable
-a. On the other hand:
+is OK because the constraint (Collection a b) mentions
+b, even though it also mentions the class variable
+a. On the other hand:
-
+
class C a where
- op :: Eq a => (a,b) -> (a,b)
-
+ op :: Eq a => (a,b) -> (a,b)
+
-is not OK because the constraint (Eq a) mentions on the class
-type variable a, but not b. However, any such
+is not OK because the constraint (Eq a) mentions on the class
+type variable a, but not b. However, any such
example is easily fixed by moving the offending context up to the
superclass context:
-
- class Eq a => C a where
- op ::(a,b) -> (a,b)
-
+
+ class Eq a => C a where
+ op ::(a,b) -> (a,b)
+
A yet more relaxed rule would allow the context of a class-op signature
to mention only class type variables. However, that conflicts with
Rule 1(b) for types above.
-
-
-
+
+
+
-
- The type of each class operation must mention all of
-the class type variables. For example:
+
+ The type of each class operation must mention all of
+the class type variables. For example:
-
+
class Coll s a where
empty :: s
- insert :: s -> a -> s
-
+ insert :: s -> a -> s
+
-is not OK, because the type of empty doesn't mention
-a. This rule is a consequence of Rule 1(a), above, for
+is not OK, because the type of empty doesn't mention
+a. This rule is a consequence of Rule 1(a), above, for
types, and has the same motivation.
Sometimes, offending class declarations exhibit misunderstandings. For
-example, Coll might be rewritten
+example, Coll might be rewritten
-
+
class Coll s a where
empty :: s a
- insert :: s a -> a -> s a
-
+ insert :: s a -> a -> s a
+
which makes the connection between the type of a collection of
-a's (namely (s a)) and the element type a.
+a's (namely (s a)) and the element type a.
Occasionally this really doesn't work, in which case you can split the
class like this:
-
+
class CollE s where
empty :: s
- class CollE s => Coll s a where
- insert :: s -> a -> s
-
+ class CollE s => Coll s a where
+ insert :: s -> a -> s
+
-
-
+
+
-
+
-
+
-
-Instance declarations
+
+Instance declarations
-
+
-
+
-
- Instance declarations may not overlap. The two instance
+
+ Instance declarations may not overlap. The two instance
declarations
-
- instance context1 => C type1 where ...
- instance context2 => C type2 where ...
-
+
+ instance context1 => C type1 where ...
+ instance context2 => C type2 where ...
+
-"overlap" if type1 and type2 unify
+"overlap" if type1 and type2 unify
However, if you give the command line option
--fallow-overlapping-instances-fallow-overlapping-instances
-option then two overlapping instance declarations are permitted
+-fallow-overlapping-instances
+option then two overlapping instance declarations are permitted
iff
-
-
+
+
-
- EITHER type1 and type2 do not unify
-
-
-
+
+ EITHER type1 and type2 do not unify
+
+
+
-
- OR type2 is a substitution instance of type1
-(but not identical to type1)
-
-
-
+
+ OR type2 is a substitution instance of type1
+(but not identical to type1)
+
+
+
-
+
OR vice versa
-
-
+
+
-
+
Notice that these rules
-
-
+
+
-
+
make it clear which instance decl to use
(pick the most specific one that matches)
-
-
-
+
+
+
-
- do not mention the contexts context1, context2
+
+ do not mention the contexts context1, context2
Reason: you can pick which instance decl
"matches" based on the type.
-
-
+
+
-
+
Regrettably, GHC doesn't guarantee to detect overlapping instance
declarations if they appear in different modules. GHC can "see" the
instance declarations in the transitive closure of all the modules
imported by the one being compiled, so it can "see" all instance decls
-when it is compiling Main. However, it currently chooses not
+when it is compiling Main. However, it currently chooses not
to look at ones that can't possibly be of use in the module currently
being compiled, in the interests of efficiency. (Perhaps we should
-change that decision, at least for Main.)
+change that decision, at least for Main.)
-
-
-
+
+
+
-
- There are no restrictions on the type in an instance
-head, except that at least one must not be a type variable.
-The instance "head" is the bit after the "=>" in an instance decl. For
+
+ There are no restrictions on the type in an instance
+head, except that at least one must not be a type variable.
+The instance "head" is the bit after the "=>" in an instance decl. For
example, these are OK:
-
+
instance C Int a where ...
instance D (Int, Int) where ...
instance E [[a]] where ...
-
+
-Note that instance heads may contain repeated type variables.
+Note that instance heads may contain repeated type variables.
For example, this is OK:
-
+
instance Stateful (ST s) (MutVar s) where ...
-
+
The "at least one not a type variable" restriction is to ensure that
@@ -1576,9 +1044,9 @@ constructor. For example, the following would make the type checker
loop if it wasn't excluded:
-
- instance C a => C a where ...
-
+
+ instance C a => C a where ...
+
There are two situations in which the rule is a bit of a pain. First,
@@ -1587,68 +1055,68 @@ convenient to have a "default instance" declaration that applies if
something more specific does not:
-
+
instance C a where
op = ... -- Default
-
+
Second, sometimes you might want to use the following to get the
effect of a "class synonym":
-
- class (C1 a, C2 a, C3 a) => C a where { }
+
+ class (C1 a, C2 a, C3 a) => C a where { }
- instance (C1 a, C2 a, C3 a) => C a where { }
-
+ instance (C1 a, C2 a, C3 a) => C a where { }
+
This allows you to write shorter signatures:
-
- f :: C a => ...
-
+
+ f :: C a => ...
+
instead of
-
- f :: (C1 a, C2 a, C3 a) => ...
-
+
+ f :: (C1 a, C2 a, C3 a) => ...
+
I'm on the lookout for a simple rule that preserves decidability while
allowing these idioms. The experimental flag
--fallow-undecidable-instances-fallow-undecidable-instances
-option lifts this restriction, allowing all the types in an
+-fallow-undecidable-instances
+option lifts this restriction, allowing all the types in an
instance head to be type variables.
-
-
-
+
+
+
-
- Unlike Haskell 1.4, instance heads may use type
-synonyms. As always, using a type synonym is just shorthand for
+
+ Unlike Haskell 1.4, instance heads may use type
+synonyms. As always, using a type synonym is just shorthand for
writing the RHS of the type synonym definition. For example:
-
+
type Point = (Int,Int)
instance C Point where ...
instance C [Point] where ...
-
+
is legal. However, if you added
-
+
instance C (Int,Int) where ...
-
+
as well, then the compiler will complain about the overlapping
@@ -1656,35 +1124,35 @@ as well, then the compiler will complain about the overlapping
must be fully applied. You cannot, for example, write:
-
+
type P a = [[a]]
instance Monad P where ...
-
+
This design decision is independent of all the others, and easily
reversed, but it makes sense to me.
-
-
-
+
+
+
-
-The types in an instance-declaration context must all
-be type variables. Thus
+
+The types in an instance-declaration context must all
+be type variables. Thus
-
-instance C a b => Eq (a,b) where ...
-
+
+instance C a b => Eq (a,b) where ...
+
is OK, but
-
-instance C Int b => Foo b where ...
-
+
+instance C Int b => Foo b where ...
+
is not OK. Again, the intent here is to make sure that context
@@ -1692,649 +1160,747 @@ reduction terminates.
Voluminous correspondence on the Haskell mailing list has convinced me
that it's worth experimenting with a more liberal rule. If you use
-the flag -fallow-undecidable-instances you can use arbitrary
+the flag can use arbitrary
types in an instance context. Termination is ensured by having a
fixed-depth recursion stack. If you exceed the stack depth you get a
sort of backtrace, and the opportunity to increase the stack depth
-with -fcontext-stackN.
+with N.
-
-
+
+
-
+
+
+
+
+
+
+
+Implicit parameters
+
+
+ Implicit paramters are implemented as described in
+"Implicit parameters: dynamic scoping with static types",
+J Lewis, MB Shields, E Meijer, J Launchbury,
+27th ACM Symposium on Principles of Programming Languages (POPL'00),
+Boston, Jan 2000.
+
+
+
+There should be more documentation, but there isn't (yet). Yell if you need it.
+
+
+
+ You can't have an implicit parameter in the context of a class or instance
+declaration. For example, both these declarations are illegal:
+
+ class (?x::Int) => C a where ...
+ instance (?x::a) => Foo [a] where ...
+
+Reason: exactly which implicit parameter you pick up depends on exactly where
+you invoke a function. But the ``invocation'' of instance declarations is done
+behind the scenes by the compiler, so it's hard to figure out exactly where it is done.
+Easiest thing is to outlaw the offending types.
+
+
+
+
+
-
-
+
+Functional dependencies
+
-
-Explicit universal quantification
-
+ Functional dependencies are implemented as described by Mark Jones
+in "Type Classes with Functional Dependencies", Mark P. Jones,
+In Proceedings of the 9th European Symposium on Programming,
+ESOP 2000, Berlin, Germany, March 2000, Springer-Verlag LNCS 1782.
+
-
-GHC now allows you to write explicitly quantified types. GHC's
-syntax for this now agrees with Hugs's, namely:
-
+
+There should be more documentation, but there isn't (yet). Yell if you need it.
+
+
-
-
- forall a b. (Ord a, Eq b) => a -> b -> a
-
+
+Explicit universal quantification
+
-
+
+GHC's type system supports explicit universal quantification in
+constructor fields and function arguments. This is useful for things
+like defining runST from the state-thread world.
+GHC's syntax for this now agrees with Hugs's, namely:
+
+
+
-
-The context is, of course, optional. You can't use forall as
+
+ forall a b. (Ord a, Eq b) => a -> b -> a
+
+
+
+
+
+The context is, of course, optional. You can't use forall as
a type variable any more!
-
+
-
-Haskell type signatures are implicitly quantified. The forall
+
+Haskell type signatures are implicitly quantified. The forall
allows us to say exactly what this means. For example:
-
+
-
+
-
- g :: b -> b
-
+
+ g :: b -> b
+
-
+
-
+
means this:
-
+
-
+
-
- g :: forall b. (b -> b)
-
+
+ g :: forall b. (b -> b)
+
-
+
-
+
The two are treated identically.
-
+
-
-Universally-quantified data type fields
-
+
+Universally-quantified data type fields
+
-
-In a data or newtype declaration one can quantify
+
+In a data or newtype declaration one can quantify
the types of the constructor arguments. Here are several examples:
-
+
-
+
-
-data T a = T1 (forall b. b -> b -> b) a
+
+data T a = T1 (forall b. b -> b -> b) a
-data MonadT m = MkMonad { return :: forall a. a -> m a,
- bind :: forall a b. m a -> (a -> m b) -> m b
+data MonadT m = MkMonad { return :: forall a. a -> m a,
+ bind :: forall a b. m a -> (a -> m b) -> m b
}
-newtype Swizzle = MkSwizzle (Ord a => [a] -> [a])
-
+newtype Swizzle = MkSwizzle (Ord a => [a] -> [a])
+
-
+
-
-The constructors now have so-called rank 2 polymorphic
+
+The constructors now have so-called rank 2 polymorphic
types, in which there is a for-all in the argument types.:
-
+
-
+
-
-T1 :: forall a. (forall b. b -> b -> b) -> a -> T1 a
-MkMonad :: forall m. (forall a. a -> m a)
- -> (forall a b. m a -> (a -> m b) -> m b)
- -> MonadT m
-MkSwizzle :: (Ord a => [a] -> [a]) -> Swizzle
-
+
+T1 :: forall a. (forall b. b -> b -> b) -> a -> T a
+MkMonad :: forall m. (forall a. a -> m a)
+ -> (forall a b. m a -> (a -> m b) -> m b)
+ -> MonadT m
+MkSwizzle :: (Ord a => [a] -> [a]) -> Swizzle
+
-
+
-
-Notice that you don't need to use a forall if there's an
+
+Notice that you don't need to use a forall if there's an
explicit context. For example in the first argument of the
-constructor MkSwizzle, an implicit "forall a." is
-prefixed to the argument type. The implicit forall
+constructor MkSwizzle, an implicit "forall a." is
+prefixed to the argument type. The implicit forall
quantifies all type variables that are not already in scope, and are
mentioned in the type quantified over.
-
+
-
+
As for type signatures, implicit quantification happens for non-overloaded
types too. So if you write this:
-
- data T a = MkT (Either a b) (b -> b)
-
+
+ data T a = MkT (Either a b) (b -> b)
+
it's just as if you had written this:
-
- data T a = MkT (forall b. Either a b) (forall b. b -> b)
-
+
+ data T a = MkT (forall b. Either a b) (forall b. b -> b)
+
-That is, since the type variable b isn't in scope, it's
+That is, since the type variable b isn't in scope, it's
implicitly universally quantified. (Arguably, it would be better
-to require explicit quantification on constructor arguments
+to require explicit quantification on constructor arguments
where that is what is wanted. Feedback welcomed.)
-
+
-
+
-
-Construction
+
+Construction
-
-You construct values of types T1, MonadT, Swizzle by applying
+
+You construct values of types T1, MonadT, Swizzle by applying
the constructor to suitable values, just as usual. For example,
-
+
-
+
-
-(T1 (\xy->x) 3) :: T Int
+
+(T1 (\xy->x) 3) :: T Int
(MkSwizzle sort) :: Swizzle
(MkSwizzle reverse) :: Swizzle
(let r x = Just x
b m k = case m of
- Just y -> k y
- Nothing -> Nothing
+ Just y -> k y
+ Nothing -> Nothing
in
MkMonad r b) :: MonadT Maybe
-
+
-
+
-
+
The type of the argument can, as usual, be more general than the type
-required, as (MkSwizzle reverse) shows. (reverse
-does not need the Ord constraint.)
-
+required, as (MkSwizzle reverse) shows. (reverse
+does not need the Ord constraint.)
+
-
+
-
-Pattern matching
+
+Pattern matching
-
+
When you use pattern matching, the bound variables may now have
polymorphic types. For example:
-
+
-
+
-
- f :: T a -> a -> (a, Char)
+
+ f :: T a -> a -> (a, Char)
f (T1 f k) x = (f k x, f 'c' 'd')
- g :: (Ord a, Ord b) => Swizzle -> [a] -> (a -> b) -> [b]
+ g :: (Ord a, Ord b) => Swizzle -> [a] -> (a -> b) -> [b]
g (MkSwizzle s) xs f = s (map f (s xs))
- h :: MonadT m -> [m a] -> m [a]
+ h :: MonadT m -> [m a] -> m [a]
h m [] = return m []
- h m (x:xs) = bind m x $ \y ->
- bind m (h m xs) $ \ys ->
+ h m (x:xs) = bind m x $ \y ->
+ bind m (h m xs) $ \ys ->
return m (y:ys)
-
+
-
+
-
-In the function h we use the record selectors return
-and bind to extract the polymorphic bind and return functions
-from the MonadT data structure, rather than using pattern
+
+In the function h we use the record selectors return
+and bind to extract the polymorphic bind and return functions
+from the MonadT data structure, rather than using pattern
matching.
-
+
-
+
You cannot pattern-match against an argument that is polymorphic.
For example:
-
+
newtype TIM s a = TIM (ST s (Maybe a))
- runTIM :: (forall s. TIM s a) -> Maybe a
+ runTIM :: (forall s. TIM s a) -> Maybe a
runTIM (TIM m) = runST m
-
+
-
+
-
+
Here the pattern-match fails, because you can't pattern-match against
-an argument of type (forall s. TIM s a). Instead you
+an argument of type (forall s. TIM s a). Instead you
must bind the variable and pattern match in the right hand side:
-
- runTIM :: (forall s. TIM s a) -> Maybe a
- runTIM tm = case tm of { TIM m -> runST m }
-
+
+ runTIM :: (forall s. TIM s a) -> Maybe a
+ runTIM tm = case tm of { TIM m -> runST m }
+
-The tm on the right hand side is (invisibly) instantiated, like
+The tm on the right hand side is (invisibly) instantiated, like
any polymorphic value at its occurrence site, and now you can pattern-match
against it.
-
+
-
+
-
-The partial-application restriction
+
+The partial-application restriction
-
+
There is really only one way in which data structures with polymorphic
components might surprise you: you must not partially apply them.
For example, this is illegal:
-
+
-
+
-
+
map MkSwizzle [sort, reverse]
-
+
-
+
-
-The restriction is this: every subexpression of the program must
+
+The restriction is this: every subexpression of the program must
have a type that has no for-alls, except that in a function
application (f e1…en) the partial applications are not subject to
-this rule. The restriction makes type inference feasible.
-
+this rule. The restriction makes type inference feasible.
+
-
-In the illegal example, the sub-expression MkSwizzle has the
-polymorphic type (Ord b => [b] -> [b]) -> Swizzle and is not
+
+In the illegal example, the sub-expression MkSwizzle has the
+polymorphic type (Ord b => [b] -> [b]) -> Swizzle and is not
a sub-expression of an enclosing application. On the other hand, this
expression is OK:
-
+
-
+
-
- map (T1 (\a b -> a)) [1,2,3]
-
+
+ map (T1 (\a b -> a)) [1,2,3]
+
-
+
-
-even though it involves a partial application of T1, because
-the sub-expression T1 (\a b -> a) has type Int -> T
-Int.
-
+
+even though it involves a partial application of T1, because
+the sub-expression T1 (\a b -> a) has type Int -> T
+Int.
+
-
+
-
-Type signatures
-
+
+Type signatures
+
-
+
Once you have data constructors with universally-quantified fields, or
-constants such as runST that have rank-2 types, it isn't long
+constants such as runST that have rank-2 types, it isn't long
before you discover that you need more! Consider:
-
+
-
+
-
+
mkTs f x y = [T1 f x, T1 f y]
-
+
-
+
-
-mkTs is a fuction that constructs some values of type
-T, using some pieces passed to it. The trouble is that since
-f is a function argument, Haskell assumes that it is
-monomorphic, so we'll get a type error when applying T1 to
+
+mkTs is a fuction that constructs some values of type
+T, using some pieces passed to it. The trouble is that since
+f is a function argument, Haskell assumes that it is
+monomorphic, so we'll get a type error when applying T1 to
it. This is a rather silly example, but the problem really bites in
practice. Lots of people trip over the fact that you can't make
-"wrappers functions" for runST for exactly the same reason.
+"wrappers functions" for runST for exactly the same reason.
In short, it is impossible to build abstractions around functions with
rank-2 types.
-
+
-
+
The solution is fairly clear. We provide the ability to give a rank-2
-type signature for ordinary functions (not only data
+type signature for ordinary functions (not only data
constructors), thus:
-
+
-
+
-
- mkTs :: (forall b. b -> b -> b) -> a -> [T a]
+
+ mkTs :: (forall b. b -> b -> b) -> a -> [T a]
mkTs f x y = [T1 f x, T1 f y]
-
+
-
+
-
-This type signature tells the compiler to attribute f with
-the polymorphic type (forall b. b -> b -> b) when type
-checking the body of mkTs, so now the application of
-T1 is fine.
-
+
+This type signature tells the compiler to attribute f with
+the polymorphic type (forall b. b -> b -> b) when type
+checking the body of mkTs, so now the application of
+T1 is fine.
+
-
+
There are two restrictions:
-
+
-
+
-
-
+
+
-
+
You can only define a rank 2 type, specified by the following
grammar:
-
- rank2type ::= [forall tyvars .] [context =>] funty
- funty ::= ([forall tyvars .] [context =>] ty) -> funty
- | ty
- ty ::= ...current Haskell monotype syntax...
-
+
+rank2type ::= [forall tyvars .] [context =>] funty
+funty ::= ([forall tyvars .] [context =>] ty) -> funty
+ | ty
+ty ::= ...current Haskell monotype syntax...
+
Informally, the universal quantification must all be right at the beginning,
or at the top level of a function argument.
-
-
-
+
+
+
-
+
There is a restriction on the definition of a function whose
type signature is a rank-2 type: the polymorphic arguments must be
-matched on the left hand side of the "=" sign. You can't
-define mkTs like this:
+matched on the left hand side of the "=" sign. You can't
+define mkTs like this:
-
- mkTs :: (forall b. b -> b -> b) -> a -> [T a]
- mkTs = \ f x y -> [T1 f x, T1 f y]
-
+
+mkTs :: (forall b. b -> b -> b) -> a -> [T a]
+mkTs = \ f x y -> [T1 f x, T1 f y]
+
The same partial-application rule applies to ordinary functions with
rank-2 types as applied to data constructors.
-
-
-
-
-
-
-
-
-
-
-
-
-Existentially quantified data constructors
-
-
-
+
+
+
+
+
+
+
+
+
+
+
+Type synonyms and hoisting
+
+
+
+GHC also allows you to write a forall in a type synonym, thus:
+
+ type Discard a = forall b. a -> b -> a
+
+ f :: Discard a
+ f x y = x
+
+However, it is often convenient to use these sort of synonyms at the right hand
+end of an arrow, thus:
+
+ type Discard a = forall b. a -> b -> a
+
+ g :: Int -> Discard Int
+ g x y z = x+y
+
+Simply expanding the type synonym would give
+
+ g :: Int -> (forall b. Int -> b -> Int)
+
+but GHC "hoists" the forall to give the isomorphic type
+
+ g :: forall b. Int -> Int -> b -> Int
+
+In general, the rule is this: to determine the type specified by any explicit
+user-written type (e.g. in a type signature), GHC expands type synonyms and then repeatedly
+performs the transformation:
+
+ type1 -> forall a. type2
+==>
+ forall a. type1 -> type2
+
+(In fact, GHC tries to retain as much synonym information as possible for use in
+error messages, but that is a usability issue.) This rule applies, of course, whether
+or not the forall comes from a synonym. For example, here is another
+valid way to write g's type signature:
+
+ g :: Int -> Int -> forall b. b -> Int
+
+
+
+
+
+
+
+Existentially quantified data constructors
+
+
+
The idea of using existential quantification in data type declarations
was suggested by Laufer (I believe, thought doubtless someone will
correct me), and implemented in Hope+. It's been in Lennart
-Augustsson's hbc Haskell compiler for several years, and
+Augustsson's hbc Haskell compiler for several years, and
proved very useful. Here's the idea. Consider the declaration:
-
+
-
+
-
- data Foo = forall a. MkFoo a (a -> Bool)
+
+ data Foo = forall a. MkFoo a (a -> Bool)
| Nil
-
+
-
+
-
-The data type Foo has two constructors with types:
-
+
+The data type Foo has two constructors with types:
+
-
+
-
- MkFoo :: forall a. a -> (a -> Bool) -> Foo
+
+ MkFoo :: forall a. a -> (a -> Bool) -> Foo
Nil :: Foo
-
+
-
+
-
-Notice that the type variable a in the type of MkFoo
-does not appear in the data type itself, which is plain Foo.
+
+Notice that the type variable a in the type of MkFoo
+does not appear in the data type itself, which is plain Foo.
For example, the following expression is fine:
-
+
-
+
-
+
[MkFoo 3 even, MkFoo 'c' isUpper] :: [Foo]
-
+
-
+
-
-Here, (MkFoo 3 even) packages an integer with a function
-even that maps an integer to Bool; and MkFoo 'c'
-isUpper packages a character with a compatible function. These
-two things are each of type Foo and can be put in a list.
-
+
+Here, (MkFoo 3 even) packages an integer with a function
+even that maps an integer to Bool; and MkFoo 'c'
+isUpper packages a character with a compatible function. These
+two things are each of type Foo and can be put in a list.
+
-
-What can we do with a value of type Foo?. In particular,
-what happens when we pattern-match on MkFoo?
-
+
+What can we do with a value of type Foo?. In particular,
+what happens when we pattern-match on MkFoo?
+
-
+
-
+
f (MkFoo val fn) = ???
-
+
-
+
-
-Since all we know about val and fn is that they
+
+Since all we know about val and fn is that they
are compatible, the only (useful) thing we can do with them is to
-apply fn to val to get a boolean. For example:
-
+apply fn to val to get a boolean. For example:
+
-
+
-
- f :: Foo -> Bool
+
+ f :: Foo -> Bool
f (MkFoo val fn) = fn val
-
+
-
+
-
+
What this allows us to do is to package heterogenous values
together with a bunch of functions that manipulate them, and then treat
that collection of packages in a uniform manner. You can express
quite a bit of object-oriented-like programming this way.
-
+
-
-Why existential?
-
+
+Why existential?
+
-
-What has this to do with existential quantification?
-Simply that MkFoo has the (nearly) isomorphic type
-
+
+What has this to do with existential quantification?
+Simply that MkFoo has the (nearly) isomorphic type
+
-
+
-
- MkFoo :: (exists a . (a, a -> Bool)) -> Foo
-
+
+ MkFoo :: (exists a . (a, a -> Bool)) -> Foo
+
-
+
-
+
But Haskell programmers can safely think of the ordinary
-universally quantified type given above, thereby avoiding
+universally quantified type given above, thereby avoiding
adding a new existential quantification construct.
-
+
-
+
-
-Type classes
+
+Type classes
-
-An easy extension (implemented in hbc) is to allow
+
+An easy extension (implemented in hbc) is to allow
arbitrary contexts before the constructor. For example:
-
+
-
+
-
- data Baz = forall a. Eq a => Baz1 a a
- | forall b. Show b => Baz2 b (b -> b)
-
+
+data Baz = forall a. Eq a => Baz1 a a
+ | forall b. Show b => Baz2 b (b -> b)
+
-
+
-
+
The two constructors have the types you'd expect:
-
+
-
+
-
- Baz1 :: forall a. Eq a => a -> a -> Baz
- Baz2 :: forall b. Show b => b -> (b -> b) -> Baz
-
+
+Baz1 :: forall a. Eq a => a -> a -> Baz
+Baz2 :: forall b. Show b => b -> (b -> b) -> Baz
+
-
+
-
-But when pattern matching on Baz1 the matched values can be compared
-for equality, and when pattern matching on Baz2 the first matched
+
+But when pattern matching on Baz1 the matched values can be compared
+for equality, and when pattern matching on Baz2 the first matched
value can be converted to a string (as well as applying the function to it).
So this program is legal:
-
+
-
+
-
- f :: Baz -> String
+
+ f :: Baz -> String
f (Baz1 p q) | p == q = "Yes"
| otherwise = "No"
f (Baz1 v fn) = show (fn v)
-
+
-
+
-
+
Operationally, in a dictionary-passing implementation, the
-constructors Baz1 and Baz2 must store the
-dictionaries for Eq and Show respectively, and
+constructors Baz1 and Baz2 must store the
+dictionaries for Eq and Show respectively, and
extract it on pattern matching.
-
+
-
+
Notice the way that the syntax fits smoothly with that used for
universal quantification earlier.
-
+
-
+
-
-Restrictions
+
+Restrictions
-
+
There are several restrictions on the ways in which existentially-quantified
constructors can be use.
-
+
-
+
-
-
+
+
-
+
When pattern matching, each pattern match introduces a new,
distinct, type for each existential type variable. These types cannot
be unified with any other type, nor can they escape from the scope of
the pattern match. For example, these fragments are incorrect:
-
- f1 (MkFoo a f) = a
-
+
+f1 (MkFoo a f) = a
+
-Here, the type bound by MkFoo "escapes", because a
-is the result of f1. One way to see why this is wrong is to
-ask what type f1 has:
+Here, the type bound by MkFoo "escapes", because a
+is the result of f1. One way to see why this is wrong is to
+ask what type f1 has:
-
- f1 :: Foo -> a -- Weird!
-
+
+ f1 :: Foo -> a -- Weird!
+
-What is this "a" in the result type? Clearly we don't mean
+What is this "a" in the result type? Clearly we don't mean
this:
-
- f1 :: forall a. Foo -> a -- Wrong!
-
+
+ f1 :: forall a. Foo -> a -- Wrong!
+
The original program is just plain wrong. Here's another sort of error
-
+
f2 (Baz1 a b) (Baz1 p q) = a==q
-
+
-It's ok to say a==b or p==q, but
-a==q is wrong because it equates the two distinct types arising
-from the two Baz1 constructors.
+It's ok to say a==b or p==q, but
+a==q is wrong because it equates the two distinct types arising
+from the two Baz1 constructors.
-
-
-
+
+
+
-
+
You can't pattern-match on an existentially quantified
-constructor in a let or where group of
+constructor in a let or where group of
bindings. So this is illegal:
-
+
f3 x = a==b where { Baz1 a b = x }
-
+
You can only pattern-match
-on an existentially-quantified constructor in a case expression or
+on an existentially-quantified constructor in a case expression or
in the patterns of a function definition.
The reason for this restriction is really an implementation one.
@@ -2345,549 +1911,486 @@ not clear how to prevent the existentially-quantified type "escaping".
So for now, there's a simple-to-state restriction. We'll see how
annoying it is.
-
-
-
+
+
+
-
-You can't use existential quantification for newtype
+
+You can't use existential quantification for newtype
declarations. So this is illegal:
-
- newtype T = forall a. Ord a => MkT a
-
+
+ newtype T = forall a. Ord a => MkT a
+
-Reason: a value of type T must be represented as a pair
-of a dictionary for Ord t and a value of type t.
-That contradicts the idea that newtype should have no
+Reason: a value of type T must be represented as a pair
+of a dictionary for Ord t and a value of type t.
+That contradicts the idea that newtype should have no
concrete representation. You can get just the same efficiency and effect
-by using data instead of newtype. If there is no
+by using data instead of newtype. If there is no
overloading involved, then there is more of a case for allowing
-an existentially-quantified newtype, because the data
-because the data version does carry an implementation cost,
+an existentially-quantified newtype, because the data
+because the data version does carry an implementation cost,
but single-field existentially quantified constructors aren't much
-use. So the simple restriction (no existential stuff on newtype)
+use. So the simple restriction (no existential stuff on newtype)
stands, unless there are convincing reasons to change it.
-
-
-
+
+
+
-
- You can't use deriving to define instances of a
+
+ You can't use deriving to define instances of a
data type with existentially quantified data constructors.
Reason: in most cases it would not make sense. For example:#
-
- data T = forall a. MkT [a] deriving( Eq )
-
+
+data T = forall a. MkT [a] deriving( Eq )
+
-To derive Eq in the standard way we would need to have equality
-between the single component of two MkT constructors:
+To derive Eq in the standard way we would need to have equality
+between the single component of two MkT constructors:
-
- instance Eq T where
- (MkT a) == (MkT b) = ???
-
+
+instance Eq T where
+ (MkT a) == (MkT b) = ???
+
-But a and b have distinct types, and so can't be compared.
+But a and b have distinct types, and so can't be compared.
It's just about possible to imagine examples in which the derived instance
would make sense, but it seems altogether simpler simply to prohibit such
declarations. Define your own instances!
-
-
+
+
-
+
-
+
-
+
-
+
-
-Assertions
-<IndexTerm><Primary>Assertions</Primary></IndexTerm>
-
+
+Assertions
+<indexterm><primary>Assertions</primary></indexterm>
+
-
+
If you want to make use of assertions in your standard Haskell code, you
could define a function like the following:
-
+
-
+
-
-assert :: Bool -> a -> a
+
+assert :: Bool -> a -> a
assert False x = error "assertion failed!"
assert _ x = x
-
+
-
+
-
+
which works, but gives you back a less than useful error message --
an assertion failed, but which and where?
-
+
-
-One way out is to define an extended assert function which also
+
+One way out is to define an extended assert function which also
takes a descriptive string to include in the error message and
perhaps combine this with the use of a pre-processor which inserts
-the source location where assert was used.
-
+the source location where assert was used.
+
-
+
Ghc offers a helping hand here, doing all of this for you. For every
-use of assert in the user's source:
-
+use of assert in the user's source:
+
-
+
-
-kelvinToC :: Double -> Double
-kelvinToC k = assert (k >= 0.0) (k+273.15)
-
+
+kelvinToC :: Double -> Double
+kelvinToC k = assert (k >= 0.0) (k+273.15)
+
-
+
-
+
Ghc will rewrite this to also include the source location where the
assertion was made,
-
+
-
+
-
-assert pred val ==> assertError "Main.hs|15" pred val
-
+
+assert pred val ==> assertError "Main.hs|15" pred val
+
-
+
-
+
The rewrite is only performed by the compiler when it spots
-applications of Exception.assert, so you can still define and
-use your own versions of assert, should you so wish. If not,
-import Exception to make use assert in your code.
-
+applications of Exception.assert, so you can still define and
+use your own versions of assert, should you so wish. If not,
+import Exception to make use assert in your code.
+
-
+
To have the compiler ignore uses of assert, use the compiler option
--fignore-asserts. -fignore-asserts option That is,
-expressions of the form assert pred e will be rewritten to e.
-
+. -fignore-asserts option That is,
+expressions of the form assert pred e will be rewritten to e.
+
-
+
Assertion failures can be caught, see the documentation for the
-Hugs/GHC Exception library for information of how.
-
+Exception library ()
+for the details.
+
-
+
-
-Scoped Type Variables
-
+
+Scoped Type Variables
+
-
-A pattern type signature can introduce a scoped type
-variable. For example
-
+
+A pattern type signature can introduce a scoped type
+variable. For example
+
-
+
-
+
f (xs::[a]) = ys ++ ys
where
ys :: [a]
ys = reverse xs
-
+
-
+
-
-The pattern (xs::[a]) includes a type signature for xs.
-This brings the type variable a into scope; it scopes over
-all the patterns and right hand sides for this equation for f.
-In particular, it is in scope at the type signature for y.
-
+
+The pattern (xs::[a]) includes a type signature for xs.
+This brings the type variable a into scope; it scopes over
+all the patterns and right hand sides for this equation for f.
+In particular, it is in scope at the type signature for y.
+
-
-At ordinary type signatures, such as that for ys, any type variables
-mentioned in the type signature that are not in scope are
+
+ Pattern type signatures are completely orthogonal to ordinary, separate
+type signatures. The two can be used independently or together.
+At ordinary type signatures, such as that for ys, any type variables
+mentioned in the type signature that are not in scope are
implicitly universally quantified. (If there are no type variables in
scope, all type variables mentioned in the signature are universally
-quantified, which is just as in Haskell 98.) In this case, since a
-is in scope, it is not universally quantified, so the type of ys is
-the same as that of xs. In Haskell 98 it is not possible to declare
-a type for ys; a major benefit of scoped type variables is that
+quantified, which is just as in Haskell 98.) In this case, since a
+is in scope, it is not universally quantified, so the type of ys is
+the same as that of xs. In Haskell 98 it is not possible to declare
+a type for ys; a major benefit of scoped type variables is that
it becomes possible to do so.
-
+
-
+
Scoped type variables are implemented in both GHC and Hugs. Where the
implementations differ from the specification below, those differences
are noted.
-
+
-
+
So much for the basic idea. Here are the details.
-
+
+
+
+What a pattern type signature means
+
+A type variable brought into scope by a pattern type signature is simply
+the name for a type. The restriction they express is that all occurrences
+of the same name mean the same type. For example:
+
+ f :: [Int] -> Int -> Int
+ f (xs::[a]) (y::a) = (head xs + y) :: a
+
+The pattern type signatures on the left hand side of
+f express the fact that xs
+must be a list of things of some type a; and that y
+must have this same type. The type signature on the expression (head xs)
+specifies that this expression must have the same type a.
+There is no requirement that the type named by "a" is
+in fact a type variable. Indeed, in this case, the type named by "a" is
+Int. (This is a slight liberalisation from the original rather complex
+rules, which specified that a pattern-bound type variable should be universally quantified.)
+For example, all of these are legal:
+
+
+ t (x::a) (y::a) = x+y*2
+
+ f (x::a) (y::b) = [x,y] -- a unifies with b
-
-Scope and implicit quantification
+ g (x::a) = x + 1::Int -- a unifies with Int
-
+ h x = let k (y::a) = [x,y] -- a is free in the
+ in k x -- environment
+
+ k (x::a) True = ... -- a unifies with Int
+ k (x::Int) False = ...
+
+ w :: [b] -> [b]
+ w (x::a) = x -- a unifies with [b]
+
-
-
+
-
- All the type variables mentioned in the patterns for a single
-function definition equation, that are not already in scope,
-are brought into scope by the patterns. We describe this set as
-the type variables bound by the equation.
+
+Scope and implicit quantification
-
-
-
+
-
+
+
+
+
+All the type variables mentioned in a pattern,
+that are not already in scope,
+are brought into scope by the pattern. We describe this set as
+the type variables bound by the pattern.
+For example:
+
+ f (x::a) = let g (y::(a,b)) = fst y
+ in
+ g (x,True)
+
+The pattern (x::a) brings the type variable
+a into scope, as well as the term
+variable x. The pattern (y::(a,b))
+contains an occurrence of the already-in-scope type variable a,
+and brings into scope the type variable b.
+
+
+
+
+
The type variables thus brought into scope may be mentioned
in ordinary type signatures or pattern type signatures anywhere within
their scope.
-
-
-
+
+
-
+
+
In ordinary type signatures, any type variable mentioned in the
-signature that is in scope is not universally quantified.
+signature that is in scope is not universally quantified.
+
+
+
-
-
-
+
-
+
Ordinary type signatures do not bring any new type variables
into scope (except in the type signature itself!). So this is illegal:
-
-
- f :: a -> a
+
+ f :: a -> a
f x = x::a
-
-
+
-It's illegal because a is not in scope in the body of f,
-so the ordinary signature x::a is equivalent to x::forall a.a;
+It's illegal because a is not in scope in the body of f,
+so the ordinary signature x::a is equivalent to x::forall a.a;
and that is an incorrect typing.
-
-
-
+
+
-
+
+
There is no implicit universal quantification on pattern type
-signatures, nor may one write an explicit forall type in a pattern
+signatures, nor may one write an explicit forall type in a pattern
type signature. The pattern type signature is a monotype.
-
-
-
+
+
-
+
+
-The type variables in the head of a class or instance declaration
-scope over the methods defined in the where part. For example:
+The type variables in the head of a class or instance declaration
+scope over the methods defined in the where part. For example:
-
+
class C a where
- op :: [a] -> a
+ op :: [a] -> a
op xs = let ys::[a]
ys = reverse xs
in
head ys
-
+
(Not implemented in Hugs yet, Dec 98).
-
-
+
+
-
+
-
+
-
-
-
-Polymorphism
-
-
-
-
-
-
-
- Pattern type signatures are completely orthogonal to ordinary, separate
-type signatures. The two can be used independently or together. There is
-no scoping associated with the names of the type variables in a separate type signature.
+
+
+Result type signatures
-
- f :: [a] -> [a]
- f (xs::[b]) = reverse xs
-
+
+
+
-
-
-
-
-
- The function must be polymorphic in the type variables
-bound by all its equations. Operationally, the type variables bound
-by one equation must not:
-
-
-
-
-
-
- Be unified with a type (such as Int, or [a]).
-
-
-
-
-
- Be unified with a type variable free in the environment.
-
-
-
-
-
- Be unified with each other. (They may unify with the type variables
-bound by another equation for the same function, of course.)
-
-
-
-
-
-
-For example, the following all fail to type check:
-
-
-
- f (x::a) (y::b) = [x,y] -- a unifies with b
-
- g (x::a) = x + 1::Int -- a unifies with Int
-
- h x = let k (y::a) = [x,y] -- a is free in the
- in k x -- environment
-
- k (x::a) True = ... -- a unifies with Int
- k (x::Int) False = ...
-
- w :: [b] -> [b]
- w (x::a) = x -- a unifies with [b]
-
-
-
-
-
-
-
-
- The pattern-bound type variable may, however, be constrained
-by the context of the principal type, thus:
-
-
-
- f (x::a) (y::a) = x+y*2
-
-
-
-gets the inferred type: forall a. Num a => a -> a -> a.
-
-
-
-
-
-
-
-
-
-
-Result type signatures
-
-
-
-
-
-
-
+
The result type of a function can be given a signature,
thus:
-
+
f (x::a) :: [a] = [x,x,x]
-
+
-The final ":: [a]" after all the patterns gives a signature to the
+The final :: [a] after all the patterns gives a signature to the
result type. Sometimes this is the only way of naming the type variable
you want:
-
- f :: Int -> [a] -> [a]
- f n :: ([a] -> [a]) = let g (x::a, y::a) = (y,x)
- in \xs -> map g (reverse xs `zip` xs)
-
+
+ f :: Int -> [a] -> [a]
+ f n :: ([a] -> [a]) = let g (x::a, y::a) = (y,x)
+ in \xs -> map g (reverse xs `zip` xs)
+
-
-
+
+
-
+
-
+
-
+
Result type signatures are not yet implemented in Hugs.
-
+
-
+
-
-Pattern signatures on other constructs
+
+Where a pattern type signature can occur
-
+
+A pattern type signature can occur in any pattern, but there
+are restrictions on pattern bindings:
+
-
-
+
+
+A pattern type signature can be on an arbitrary sub-pattern, not
+ust on a variable:
-
- A pattern type signature can be on an arbitrary sub-pattern, not
-just on a variable:
-
-
+
f ((x,y)::(a,b)) = (y,x) :: (b,a)
-
+
-
-
-
+
+
+
-
+
Pattern type signatures, including the result part, can be used
in lambda abstractions:
+
+ (\ (x::a, y) :: a -> x)
+
+
+
+
-
- (\ (x::a, y) :: a -> x)
-
-
-
-Type variables bound by these patterns must be polymorphic in
-the sense defined above.
-For example:
-
-
-
- f1 (x::c) = f1 x -- ok
- f2 = \(x::c) -> f2 x -- not ok
-
-
-
-Here, f1 is OK, but f2 is not, because c gets unified
-with a type variable free in the environment, in this
-case, the type of f2, which is in the environment when
-the lambda abstraction is checked.
-
-
-
-
-
-
+
Pattern type signatures, including the result part, can be used
-in case expressions:
+in case expressions:
-
- case e of { (x::a, y) :: a -> x }
-
+
+ case e of { (x::a, y) :: a -> x }
+
+
+
-The pattern-bound type variables must, as usual,
-be polymorphic in the following sense: each case alternative,
-considered as a lambda abstraction, must be polymorphic.
-Thus this is OK:
+
+
+To avoid ambiguity, the type after the “::” in a result
+pattern signature on a lambda or case must be atomic (i.e. a single
+token or a parenthesised type of some sort). To see why,
+consider how one would parse this:
-
- case (True,False) of { (x::a, y) -> x }
-
+
+ \ x :: a -> b -> x
+
-Even though the context is that of a pair of booleans,
-the alternative itself is polymorphic. Of course, it is
-also OK to say:
+
+
+
-
- case (True,False) of { (x::Bool, y) -> x }
-
+
+ Pattern type signatures can bind existential type variables.
+For example:
-
-
-
+
+ data T = forall a. MkT [a]
-
-To avoid ambiguity, the type after the ``::'' in a result
-pattern signature on a lambda or case must be atomic (i.e. a single
-token or a parenthesised type of some sort). To see why,
-consider how one would parse this:
+ f :: T -> T
+ f (MkT [t::a]) = MkT t3
+ where
+ t3::[a] = [t,t,t]
+
-
- \ x :: a -> b -> x
-
+
+
-
-
-
+
-
- Pattern type signatures that bind new type variables
+
+Pattern type signatures that bind new type variables
may not be used in pattern bindings at all.
So this is illegal:
-
+
f x = let (y, z::a) = x in ...
-
+
But these are OK, because they do not bind fresh type variables:
-
+
f1 x = let (y, z::Int) = x in ...
f2 (x::(Int,a)) = let (y, z::a) = x in ...
-
+
However a single variable is considered a degenerate function binding,
@@ -2895,854 +2398,1133 @@ rather than a degerate pattern binding, so this is permitted, even
though it binds a type variable:
-
- f :: (b->b) = \(x::b) -> x
-
+
+ f :: (b->b) = \(x::b) -> x
+
-
-
+
+
-
+
Such degnerate function bindings do not fall under the monomorphism
restriction. Thus:
-
+
-
+
-
- g :: a -> a -> Bool = \x y. x==y
-
+
+ g :: a -> a -> Bool = \x y. x==y
+
-
+
-
-Here g has type forall a. Eq a => a -> a -> Bool, just as if
-g had a separate type signature. Lacking a type signature, g
+
+Here g has type forall a. Eq a => a -> a -> Bool, just as if
+g had a separate type signature. Lacking a type signature, g
would get a monomorphic type.
-
-
-
+
-
-Existentials
+
-
-
-
-
-
- Pattern type signatures can bind existential type variables.
-For example:
+
+
+ Pragmas
-
- data T = forall a. MkT [a]
-
- f :: T -> T
- f (MkT [t::a]) = MkT t3
- where
- t3::[a] = [t,t,t]
-
+ pragma
+ GHC supports several pragmas, or instructions to the
+ compiler placed in the source code. Pragmas don't normally affect
+ the meaning of the program, but they might affect the efficiency
+ of the generated code.
-
-
+ Pragmas all take the form
-
+{-# word ... #-}
-
+ where word indicates the type of
+ pragma, and is followed optionally by information specific to that
+ type of pragma. Case is ignored in
+ word. The various values for
+ word that GHC understands are described
+ in the following sections; any pragma encountered with an
+ unrecognised word is (silently)
+ ignored.
-
+
+INLINE pragma
-</Sect1>
+<indexterm><primary>INLINE pragma</primary></indexterm>
+<indexterm><primary>pragma, INLINE</primary></indexterm>
-
-Pragmas
-
-
-
-GHC supports several pragmas, or instructions to the compiler placed
-in the source code. Pragmas don't affect the meaning of the program,
-but they might affect the efficiency of the generated code.
-
-
-
-INLINE pragma
-
-<IndexTerm><Primary>INLINE pragma</Primary></IndexTerm>
-<IndexTerm><Primary>pragma, INLINE</Primary></IndexTerm>
-
-
-GHC (with -O, as always) tries to inline (or ``unfold'')
-functions/values that are ``small enough,'' thus avoiding the call
+
+GHC (with , as always) tries to inline (or “unfold”)
+functions/values that are “small enough,” thus avoiding the call
overhead and possibly exposing other more-wonderful optimisations.
-
+
-
+
You will probably see these unfoldings (in Core syntax) in your
interface files.
-
+
-
-Normally, if GHC decides a function is ``too expensive'' to inline, it
+
+Normally, if GHC decides a function is “too expensive” to inline, it
will not do so, nor will it export that unfolding for other modules to
use.
-
+
-
+
The sledgehammer you can bring to bear is the
-INLINEINLINE pragma pragma, used thusly:
+INLINEINLINE pragma pragma, used thusly:
-
-key_function :: Int -> String -> (Bool, Double)
+
+key_function :: Int -> String -> (Bool, Double)
#ifdef __GLASGOW_HASKELL__
{-# INLINE key_function #-}
#endif
-
+
(You don't need to do the C pre-processor carry-on unless you're going
-to stick the code through HBC—it doesn't like INLINE pragmas.)
-
+to stick the code through HBC—it doesn't like INLINE pragmas.)
+
-
-The major effect of an INLINE pragma is to declare a function's
-``cost'' to be very low. The normal unfolding machinery will then be
+
+The major effect of an INLINE pragma is to declare a function's
+“cost” to be very low. The normal unfolding machinery will then be
very keen to inline it.
-
+
-
-An INLINE pragma for a function can be put anywhere its type
+
+An INLINE pragma for a function can be put anywhere its type
signature could be put.
-
+
-
-INLINE pragmas are a particularly good idea for the
-then/return (or bind/unit) functions in a monad.
-For example, in GHC's own UniqueSupply monad code, we have:
+
+INLINE pragmas are a particularly good idea for the
+then/return (or bind/unit) functions in a monad.
+For example, in GHC's own UniqueSupply monad code, we have:
-
+
#ifdef __GLASGOW_HASKELL__
{-# INLINE thenUs #-}
{-# INLINE returnUs #-}
#endif
-
-
-
-
-
+
-
-NOINLINE pragma
-
+
-
-NOINLINE pragma
-pragma, NOINLINE
-
+
-
-The NOINLINE pragma does exactly what you'd expect: it stops the
-named function from being inlined by the compiler. You shouldn't ever
-need to do this, unless you're very cautious about code size.
-
+
+NOINLINE pragma
+
-
+NOINLINE pragma
+pragmaNOINLINE
+NOTINLINE pragma
+pragmaNOTINLINE
-
-SPECIALIZE pragma
-
+
+The NOINLINE pragma does exactly what you'd expect:
+it stops the named function from being inlined by the compiler. You
+shouldn't ever need to do this, unless you're very cautious about code
+size.
+
-
-SPECIALIZE pragma
-pragma, SPECIALIZE
-overloading, death to
-
+NOTINLINE is a synonym for
+NOINLINE (NOTINLINE is specified
+by Haskell 98 as the standard way to disable inlining, so it should be
+used if you want your code to be portable).
-
-(UK spelling also accepted.) For key overloaded functions, you can
-create extra versions (NB: more code space) specialised to particular
-types. Thus, if you have an overloaded function:
-
+
-
+
+ SPECIALIZE pragma
-
-hammeredLookup :: Ord key => [(key, value)] -> key -> value
-
+ SPECIALIZE pragma
+ pragma, SPECIALIZE
+ overloading, death to
-
+ (UK spelling also accepted.) For key overloaded
+ functions, you can create extra versions (NB: more code space)
+ specialised to particular types. Thus, if you have an
+ overloaded function:
-
-If it is heavily used on lists with Widget keys, you could
-specialise it as follows:
+
+hammeredLookup :: Ord key => [(key, value)] -> key -> value
+
-
-{-# SPECIALIZE hammeredLookup :: [(Widget, value)] -> Widget -> value #-}
-
+ If it is heavily used on lists with
+ Widget keys, you could specialise it as
+ follows:
-
+
+{-# SPECIALIZE hammeredLookup :: [(Widget, value)] -> Widget -> value #-}
+
-
-To get very fancy, you can also specify a named function to use for
-the specialised value, by adding = blah, as in:
+ To get very fancy, you can also specify a named function
+ to use for the specialised value, as in:
-
-{-# SPECIALIZE hammeredLookup :: ...as before... = blah #-}
-
+
+{-# RULES hammeredLookup = blah #-}
+
-It's Your Responsibility to make sure that blah really
-behaves as a specialised version of hammeredLookup!!!
-
+ where blah is an implementation of
+ hammerdLookup written specialy for
+ Widget lookups. It's Your
+ Responsibility to make sure that
+ blah really behaves as a specialised
+ version of hammeredLookup!!!
-
-NOTE: the =blah feature isn't implemented in GHC 4.xx.
-
+ Note we use the RULE pragma here to
+ indicate that hammeredLookup applied at a
+ certain type should be replaced by blah. See
+ for more information on
+ RULES.
-
-An example in which the = blah form will Win Big:
+ An example in which using RULES for
+ specialisation will Win Big:
-
-toDouble :: Real a => a -> Double
+
+toDouble :: Real a => a -> Double
toDouble = fromRational . toRational
-{-# SPECIALIZE toDouble :: Int -> Double = i2d #-}
+{-# SPECIALIZE toDouble :: Int -> Double = i2d #-}
i2d (I# i) = D# (int2Double# i) -- uses Glasgow prim-op directly
-
+
-The i2d function is virtually one machine instruction; the
-default conversion—via an intermediate Rational—is obscenely
-expensive by comparison.
-
+ The i2d function is virtually one machine
+ instruction; the default conversion—via an intermediate
+ Rational—is obscenely expensive by
+ comparison.
-
-By using the US spelling, your SPECIALIZE pragma will work with
-HBC, too. Note that HBC doesn't support the = blah form.
-
+ A SPECIALIZE pragma for a function can
+ be put anywhere its type signature could be put.
-
-A SPECIALIZE pragma for a function can be put anywhere its type
-signature could be put.
-
-
-
+
-
-SPECIALIZE instance pragma
-
+
+SPECIALIZE instance pragma
+
-
-SPECIALIZE pragma
-overloading, death to
+
+SPECIALIZE pragma
+overloading, death to
Same idea, except for instance declarations. For example:
-
-instance (Eq a) => Eq (Foo a) where { ... usual stuff ... }
+
+instance (Eq a) => Eq (Foo a) where { ... usual stuff ... }
{-# SPECIALIZE instance Eq (Foo [(Int, Bar)] #-}
-
+
Compatible with HBC, by the way.
-
+
-
+
-
-LINE pragma
-
+
+LINE pragma
+
-
-LINE pragma
-pragma, LINE
-
+
+LINE pragma
+pragma, LINE
+
-
-This pragma is similar to C's #line pragma, and is mainly for use in
+
+This pragma is similar to C's #line pragma, and is mainly for use in
automatically generated Haskell code. It lets you specify the line
number and filename of the original code; for example
-
+
-
+
-
+
{-# LINE 42 "Foo.vhs" #-}
-
+
-
+
-
-if you'd generated the current file from something called Foo.vhs
+
+if you'd generated the current file from something called Foo.vhs
and this line corresponds to line 42 in the original. GHC will adjust
-its error messages to refer to the line/file named in the LINE
+its error messages to refer to the line/file named in the LINE
pragma.
-
+
-
+
-
-RULES pragma
+
+RULES pragma
-
+
The RULES pragma lets you specify rewrite rules. It is described in
-.
-
-
-
-
-
-
-
-Rewrite rules
-
-<IndexTerm><Primary>RULES pagma</Primary></IndexTerm>
-<IndexTerm><Primary>pragma, RULES</Primary></IndexTerm>
-<IndexTerm><Primary>rewrite rules</Primary></IndexTerm>
-
-
+.
+
+
+
+
+
+DEPRECATED pragma
+
+
+The DEPRECATED pragma lets you specify that a particular function, class, or type, is deprecated.
+There are two forms.
+
+
+
+You can deprecate an entire module thus:
+
+ module Wibble {-# DEPRECATED "Use Wobble instead" #-} where
+ ...
+
+
+When you compile any module that import Wibble, GHC will print
+the specified message.
+
+
+
+
+You can deprecate a function, class, or type, with the following top-level declaration:
+
+
+ {-# DEPRECATED f, C, T "Don't use these" #-}
+
+
+When you compile any module that imports and uses any of the specifed entities,
+GHC will print the specified message.
+
+
+
+You can suppress the warnings with the flag .
+
+
+
+
+
+
+Rewrite rules
+
+<indexterm><primary>RULES pagma</primary></indexterm>
+<indexterm><primary>pragma, RULES</primary></indexterm>
+<indexterm><primary>rewrite rules</primary></indexterm>
+
+
The programmer can specify rewrite rules as part of the source program
(in a pragma). GHC applies these rewrite rules wherever it can.
-
+
-
+
Here is an example:
-
+
{-# RULES
"map/map" forall f g xs. map f (map g xs) = map (f.g) xs
#-}
-
+
-
+
-
-Syntax
+
+Syntax
-
+
From a syntactic point of view:
-
-
+
+
-
+
Each rule has a name, enclosed in double quotes. The name itself has
no significance at all. It is only used when reporting how many times the rule fired.
-
-
-
-
-
- There may be zero or more rules in a RULES pragma.
-
-
-
-
-
- Layout applies in a RULES pragma. Currently no new indentation level
+
+
+
+
+
+ There may be zero or more rules in a RULES pragma.
+
+
+
+
+
+ Layout applies in a RULES pragma. Currently no new indentation level
is set, so you must lay out your rules starting in the same column as the
enclosing definitions.
-
-
-
-
-
- Each variable mentioned in a rule must either be in scope (e.g. map),
-or bound by the forall (e.g. f, g, xs). The variables bound by
-the forall are called the pattern variables. They are separated
-by spaces, just like in a type forall.
-
-
-
-
-
+
+
+
+
+
+ Each variable mentioned in a rule must either be in scope (e.g. map),
+or bound by the forall (e.g. f, g, xs). The variables bound by
+the forall are called the pattern variables. They are separated
+by spaces, just like in a type forall.
+
+
+
+
+
A pattern variable may optionally have a type signature.
-If the type of the pattern variable is polymorphic, it must have a type signature.
-For example, here is the foldr/build rule:
+If the type of the pattern variable is polymorphic, it must have a type signature.
+For example, here is the foldr/build rule:
-
- "fold/build" forall k z (g::forall b. (a->b->b) -> b -> b) .
- foldr k z (build g) = g k z
-
+
+"fold/build" forall k z (g::forall b. (a->b->b) -> b -> b) .
+ foldr k z (build g) = g k z
+
-Since g has a polymorphic type, it must have a type signature.
+Since g has a polymorphic type, it must have a type signature.
-
-
-
+
+
+
-
- The left hand side of a rule must consist of a top-level variable applied
-to arbitrary expressions. For example, this is not OK:
+
+The left hand side of a rule must consist of a top-level variable applied
+to arbitrary expressions. For example, this is not OK:
-
- "wrong1" forall e1 e2. case True of { True -> e1; False -> e2 } = e1
- "wrong2" forall f. f True = True
-
+
+"wrong1" forall e1 e2. case True of { True -> e1; False -> e2 } = e1
+"wrong2" forall f. f True = True
+
-In "wrong1", the LHS is not an application; in "wrong1", the LHS has a pattern variable
+In "wrong1", the LHS is not an application; in "wrong2", the LHS has a pattern variable
in the head.
-
-
-
+
+
+
-
+
A rule does not need to be in the same module as (any of) the
variables it mentions, though of course they need to be in scope.
-
-
-
+
+
+
-
+
Rules are automatically exported from a module, just as instance declarations are.
-
-
+
+
-
+
-
+
-
+
-
-Semantics
+
+Semantics
-
+
From a semantic point of view:
-
-
-
-
- Rules are only applied if you use the -O flag.
+
+
-
-
-
+
+Rules are only applied if you use the flag.
+
+
-
+
+
Rules are regarded as left-to-right rewrite rules.
When GHC finds an expression that is a substitution instance of the LHS
of a rule, it replaces the expression by the (appropriately-substituted) RHS.
By "a substitution instance" we mean that the LHS can be made equal to the
expression by substituting for the pattern variables.
-
-
-
+
+
+
-
+
The LHS and RHS of a rule are typechecked, and must have the
same type.
-
-
-
+
+
+
-
+
GHC makes absolutely no attempt to verify that the LHS and RHS
of a rule have the same meaning. That is undecideable in general, and
infeasible in most interesting cases. The responsibility is entirely the programmer's!
-
-
-
+
+
+
-
+
GHC makes no attempt to make sure that the rules are confluent or
terminating. For example:
-
+
"loop" forall x,y. f x y = f y x
-
+
This rule will cause the compiler to go into an infinite loop.
-
-
-
+
+
+
-
+
If more than one rule matches a call, GHC will choose one arbitrarily to apply.
-
-
-
-
+
+
+
+
GHC currently uses a very simple, syntactic, matching algorithm
for matching a rule LHS with an expression. It seeks a substitution
which makes the LHS and expression syntactically equal modulo alpha
conversion. The pattern (rule), but not the expression, is eta-expanded if
necessary. (Eta-expanding the epression can lead to laziness bugs.)
But not beta conversion (that's called higher-order matching).
-
+
-
+
Matching is carried out on GHC's intermediate language, which includes
type abstractions and applications. So a rule only matches if the
-types match too. See below.
-
-
-
+types match too. See below.
+
+
+
-
+
GHC keeps trying to apply the rules as it optimises the program.
For example, consider:
-
+
let s = map f
t = map g
in
s (t xs)
-
+
-The expression s (t xs) does not match the rule "map/map", but GHC
-will substitute for s and t, giving an expression which does match.
-If s or t was (a) used more than once, and (b) large or a redex, then it would
+The expression s (t xs) does not match the rule "map/map", but GHC
+will substitute for s and t, giving an expression which does match.
+If s or t was (a) used more than once, and (b) large or a redex, then it would
not be substituted, and the rule would not fire.
-
-
-
+
+
+
-
- In the earlier phases of compilation, GHC inlines nothing
-that appears on the LHS of a rule, because once you have substituted
+
+ In the earlier phases of compilation, GHC inlines nothing
+that appears on the LHS of a rule, because once you have substituted
for something you can't match against it (given the simple minded
matching). So if you write the rule
-
+
"map/map" forall f,g. map f . map g = map (f.g)
-
+
-this won't match the expression map f (map g xs).
+this won't match the expression map f (map g xs).
It will only match something written with explicit use of ".".
-Well, not quite. It will match the expression
+Well, not quite. It will match the expression
-
- wibble f g xs
-
+
+wibble f g xs
+
-where wibble is defined:
+where wibble is defined:
-
- wibble f g = map f . map g
-
+
+wibble f g = map f . map g
+
-because wibble will be inlined (it's small).
+because wibble will be inlined (it's small).
Later on in compilation, GHC starts inlining even things on the
LHS of rules, but still leaves the rules enabled. This inlining
-policy is controlled by the per-simplification-pass flag -finline-phasen.
+policy is controlled by the per-simplification-pass flag n.
-
-
-
+
+
+
-
+
All rules are implicitly exported from the module, and are therefore
in force in any module that imports the module that defined the rule, directly
or indirectly. (That is, if A imports B, which imports C, then C's rules are
in force when compiling A.) The situation is very similar to that for instance
declarations.
-
-
+
+
-
+
-
+
-
+
-
-List fusion
+
+List fusion
-
+
The RULES mechanism is used to implement fusion (deforestation) of common list functions.
If a "good consumer" consumes an intermediate list constructed by a "good producer", the
intermediate list should be eliminated entirely.
-
+
-
+
The following are good producers:
-
-
+
+
-
+
List comprehensions
-
-
-
-
-
- Enumerations of Int and Char (e.g. ['a'..'z']).
-
-
-
-
-
- Explicit lists (e.g. [True, False])
-
-
-
-
-
- The cons constructor (e.g 3:4:[])
-
-
-
-
-
- ++
-
-
-
-
-
- map
-
-
-
-
-
- filter
-
-
-
-
-
- iterate, repeat
-
-
-
-
-
- zip, zipWith
-
-
-
-
-
-
-
-
+
+
+
+
+
+ Enumerations of Int and Char (e.g. ['a'..'z']).
+
+
+
+
+
+ Explicit lists (e.g. [True, False])
+
+
+
+
+
+ The cons constructor (e.g 3:4:[])
+
+
+
+
+
+ ++
+
+
+
+
+
+ map
+
+
+
+
+
+ filter
+
+
+
+
+
+ iterate, repeat
+
+
+
+
+
+ zip, zipWith
+
+
+
+
+
+
+
+
The following are good consumers:
-
-
+
+
-
+
List comprehensions
-
-
-
-
-
- array (on its second argument)
-
-
-
-
-
- length
-
-
-
-
-
- ++ (on its first argument)
-
-
-
-
-
- map
-
-
-
-
-
- filter
-
-
-
-
-
- concat
-
-
-
-
-
- unzip, unzip2, unzip3, unzip4
-
-
-
-
-
- zip, zipWith (but on one argument only; if both are good producers, zip
+
+
+
+
+
+ array (on its second argument)
+
+
+
+
+
+ length
+
+
+
+
+
+ ++ (on its first argument)
+
+
+
+
+
+ foldr
+
+
+
+
+
+ map
+
+
+
+
+
+ filter
+
+
+
+
+
+ concat
+
+
+
+
+
+ unzip, unzip2, unzip3, unzip4
+
+
+
+
+
+ zip, zipWith (but on one argument only; if both are good producers, zip
will fuse with one but not the other)
-
-
-
-
-
- partition
-
-
-
-
-
- head
-
-
-
-
-
- and, or, any, all
-
-
-
-
-
- sequence_
-
-
-
-
-
- msum
-
-
-
-
-
- sortBy
-
-
-
-
-
-
-
-
+
+
+
+
+
+ partition
+
+
+
+
+
+ head
+
+
+
+
+
+ and, or, any, all
+
+
+
+
+
+ sequence_
+
+
+
+
+
+ msum
+
+
+
+
+
+ sortBy
+
+
+
+
+
+
+
+
So, for example, the following should generate no intermediate lists:
-
- array (1,10) [(i,i*i) | i <- map (+ 1) [0..9]]
-
+
+array (1,10) [(i,i*i) | i <- map (+ 1) [0..9]]
+
-
+
-
+
This list could readily be extended; if there are Prelude functions that you use
a lot which are not included, please tell us.
-
+
-
+
If you want to write your own good consumers or producers, look at the
Prelude definitions of the above functions to see how to do so.
-
+
-
+
-
-Specialisation
-
+
+Specialisation
+
-
+
Rewrite rules can be used to get the same effect as a feature
present in earlier version of GHC:
-
- {-# SPECIALIZE fromIntegral :: Int8 -> Int16 = int8ToInt16 #-}
-
+
+ {-# SPECIALIZE fromIntegral :: Int8 -> Int16 = int8ToInt16 #-}
+
-This told GHC to use int8ToInt16 instead of fromIntegral whenever
-the latter was called with type Int8 -> Int16. That is, rather than
-specialising the original definition of fromIntegral the programmer is
-promising that it is safe to use int8ToInt16 instead.
-
+This told GHC to use int8ToInt16 instead of fromIntegral whenever
+the latter was called with type Int8 -> Int16. That is, rather than
+specialising the original definition of fromIntegral the programmer is
+promising that it is safe to use int8ToInt16 instead.
+
-
+
This feature is no longer in GHC. But rewrite rules let you do the
same thing:
-
- {-# RULES
- "fromIntegral/Int8/Int16" fromIntegral = int8ToInt16
- #-}
-
+
+{-# RULES
+ "fromIntegral/Int8/Int16" fromIntegral = int8ToInt16
+#-}
+
-This slightly odd-looking rule instructs GHC to replace fromIntegral
-by int8ToInt16whenever the types match. Speaking more operationally,
+This slightly odd-looking rule instructs GHC to replace fromIntegral
+by int8ToInt16whenever the types match. Speaking more operationally,
GHC adds the type and dictionary applications to get the typed rule
-
- forall (d1::Integral Int8) (d2::Num Int16) .
- fromIntegral Int8 Int16 d1 d2 = int8ToInt16
-
+
+forall (d1::Integral Int8) (d2::Num Int16) .
+ fromIntegral Int8 Int16 d1 d2 = int8ToInt16
+
What is more,
this rule does not need to be in the same file as fromIntegral,
-unlike the SPECIALISE pragmas which currently do (so that they
+unlike the SPECIALISE pragmas which currently do (so that they
have an original definition available to specialise).
-
+
-
+
-
-Controlling what's going on
+
+Controlling what's going on
-
+
-
-
+
+
-
- Use -ddump-rules to see what transformation rules GHC is using.
-
-
-
+
+ Use to see what transformation rules GHC is using.
+
+
+
-
- Use -ddump-simpl-stats to see what rules are being fired.
-If you add -dppr-debug you get a more detailed listing.
-
-
-
+
+ Use to see what rules are being fired.
+If you add you get a more detailed listing.
+
+
+
-
- The defintion of (say) build in PrelBase.lhs looks llike this:
+
+ The defintion of (say) build in PrelBase.lhs looks llike this:
-
- build :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a]
+
+ build :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a]
{-# INLINE build #-}
build g = g (:) []
-
+
-Notice the INLINE! That prevents (:) from being inlined when compiling
-PrelBase, so that an importing module will ``see'' the (:), and can
-match it on the LHS of a rule. INLINE prevents any inlining happening
-in the RHS of the INLINE thing. I regret the delicacy of this.
+Notice the INLINE! That prevents (:) from being inlined when compiling
+PrelBase, so that an importing module will “see” the (:), and can
+match it on the LHS of a rule. INLINE prevents any inlining happening
+in the RHS of the INLINE thing. I regret the delicacy of this.
-
-
-
+
+
+
-
- In ghc/lib/std/PrelBase.lhs look at the rules for map to
+
+ In ghc/lib/std/PrelBase.lhs look at the rules for map to
see how to write rules that will do fusion and yet give an efficient
-program even if fusion doesn't happen. More rules in PrelList.lhs.
-
-
-
-
-
-
-
-
-
-
+program even if fusion doesn't happen. More rules in PrelList.lhs.
+
+
+
+
+
+
+
+
+
+
+
+
+Generic classes
+
+ (Note: support for generic classes is currently broken in
+ GHC 5.02).
+
+
+The ideas behind this extension are described in detail in "Derivable type classes",
+Ralf Hinze and Simon Peyton Jones, Haskell Workshop, Montreal Sept 2000, pp94-105.
+An example will give the idea:
+
+
+
+ import Generics
+
+ class Bin a where
+ toBin :: a -> [Int]
+ fromBin :: [Int] -> (a, [Int])
+
+ toBin {| Unit |} Unit = []
+ toBin {| a :+: b |} (Inl x) = 0 : toBin x
+ toBin {| a :+: b |} (Inr y) = 1 : toBin y
+ toBin {| a :*: b |} (x :*: y) = toBin x ++ toBin y
+
+ fromBin {| Unit |} bs = (Unit, bs)
+ fromBin {| a :+: b |} (0:bs) = (Inl x, bs') where (x,bs') = fromBin bs
+ fromBin {| a :+: b |} (1:bs) = (Inr y, bs') where (y,bs') = fromBin bs
+ fromBin {| a :*: b |} bs = (x :*: y, bs'') where (x,bs' ) = fromBin bs
+ (y,bs'') = fromBin bs'
+
+
+This class declaration explains how toBin and fromBin
+work for arbitrary data types. They do so by giving cases for unit, product, and sum,
+which are defined thus in the library module Generics:
+
+
+ data Unit = Unit
+ data a :+: b = Inl a | Inr b
+ data a :*: b = a :*: b
+
+
+Now you can make a data type into an instance of Bin like this:
+
+ instance (Bin a, Bin b) => Bin (a,b)
+ instance Bin a => Bin [a]
+
+That is, just leave off the "where" clasuse. Of course, you can put in the
+where clause and over-ride whichever methods you please.
+
+
+
+ Using generics
+ To use generics you need to
+
+
+ Use the flags (to enable the extra syntax),
+ (to generate extra per-data-type code),
+ and (to make the Generics library
+ available.
+
+
+ Import the module Generics from the
+ lang package. This import brings into
+ scope the data types Unit,
+ :*:, and :+:. (You
+ don't need this import if you don't mention these types
+ explicitly; for example, if you are simply giving instance
+ declarations.)
+
+
+
+
+ Changes wrt the paper
+
+Note that the type constructors :+: and :*:
+can be written infix (indeed, you can now use
+any operator starting in a colon as an infix type constructor). Also note that
+the type constructors are not exactly as in the paper (Unit instead of 1, etc).
+Finally, note that the syntax of the type patterns in the class declaration
+uses "{|" and "|}" brackets; curly braces
+alone would ambiguous when they appear on right hand sides (an extension we
+anticipate wanting).
+
+
+
+Terminology and restrictions
+
+Terminology. A "generic default method" in a class declaration
+is one that is defined using type patterns as above.
+A "polymorphic default method" is a default method defined as in Haskell 98.
+A "generic class declaration" is a class declaration with at least one
+generic default method.
+
+
+
+Restrictions:
+
+
+
+Alas, we do not yet implement the stuff about constructor names and
+field labels.
+
+
+
+
+
+A generic class can have only one parameter; you can't have a generic
+multi-parameter class.
+
+
+
+
+
+A default method must be defined entirely using type patterns, or entirely
+without. So this is illegal:
+
+ class Foo a where
+ op :: a -> (a, Bool)
+ op {| Unit |} Unit = (Unit, True)
+ op x = (x, False)
+
+However it is perfectly OK for some methods of a generic class to have
+generic default methods and others to have polymorphic default methods.
+
+
+
+
+
+The type variable(s) in the type pattern for a generic method declaration
+scope over the right hand side. So this is legal (note the use of the type variable ``p'' in a type signature on the right hand side:
+
+ class Foo a where
+ op :: a -> Bool
+ op {| p :*: q |} (x :*: y) = op (x :: p)
+ ...
+
+
+
+
+
+
+The type patterns in a generic default method must take one of the forms:
+
+ a :+: b
+ a :*: b
+ Unit
+
+where "a" and "b" are type variables. Furthermore, all the type patterns for
+a single type constructor (:*:, say) must be identical; they
+must use the same type variables. So this is illegal:
+
+ class Foo a where
+ op :: a -> Bool
+ op {| a :+: b |} (Inl x) = True
+ op {| p :+: q |} (Inr y) = False
+
+The type patterns must be identical, even in equations for different methods of the class.
+So this too is illegal:
+
+ class Foo a where
+ op1 :: a -> Bool
+ op1 {| a :*: b |} (x :*: y) = True
+
+ op2 :: a -> Bool
+ op2 {| p :*: q |} (x :*: y) = False
+
+(The reason for this restriction is that we gather all the equations for a particular type consructor
+into a single generic instance declaration.)
+
+
+
+
+
+A generic method declaration must give a case for each of the three type constructors.
+
+
+
+
+
+The type for a generic method can be built only from:
+
+ Function arrows
+ Type variables
+ Tuples
+ Arbitrary types not involving type variables
+
+Here are some example type signatures for generic methods:
+
+ op1 :: a -> Bool
+ op2 :: Bool -> (a,Bool)
+ op3 :: [Int] -> a -> a
+ op4 :: [a] -> Bool
+
+Here, op1, op2, op3 are OK, but op4 is rejected, because it has a type variable
+inside a list.
+
+
+This restriction is an implementation restriction: we just havn't got around to
+implementing the necessary bidirectional maps over arbitrary type constructors.
+It would be relatively easy to add specific type constructors, such as Maybe and list,
+to the ones that are allowed.
+
+
+
+
+In an instance declaration for a generic class, the idea is that the compiler
+will fill in the methods for you, based on the generic templates. However it can only
+do so if
+
+
+
+ The instance type is simple (a type constructor applied to type variables, as in Haskell 98).
+
+
+
+
+ No constructor of the instance type has unboxed fields.
+
+
+
+(Of course, these things can only arise if you are already using GHC extensions.)
+However, you can still give an instance declarations for types which break these rules,
+provided you give explicit code to override any generic default methods.
+
+
+
+
+
+
+
+The option dumps incomprehensible stuff giving details of
+what the compiler does with generic declarations.
+
+
+
+
+ Another example
+
+Just to finish with, here's another example I rather like:
+
+ class Tag a where
+ nCons :: a -> Int
+ nCons {| Unit |} _ = 1
+ nCons {| a :*: b |} _ = 1
+ nCons {| a :+: b |} _ = nCons (bot::a) + nCons (bot::b)
+
+ tag :: a -> Int
+ tag {| Unit |} _ = 1
+ tag {| a :*: b |} _ = 1
+ tag {| a :+: b |} (Inl x) = tag x
+ tag {| a :+: b |} (Inr y) = nCons (bot::a) + tag y
+
+
+
+
+
+