X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fdocs%2Fusers_guide%2Fglasgow_exts.xml;h=1e0e5b94c7069473afe37eb9f8650b2e434ee2cc;hb=0f965da50e02edda7cd4e441dfdc379ba77f5d98;hp=0e19d422e8457681479e27fb84e991b5ddda44c2;hpb=55d795e2ca8f0c97e3002d4eb528862dd9d44e55;p=ghc-hetmet.git
diff --git a/ghc/docs/users_guide/glasgow_exts.xml b/ghc/docs/users_guide/glasgow_exts.xml
index 0e19d42..1e0e5b9 100644
--- a/ghc/docs/users_guide/glasgow_exts.xml
+++ b/ghc/docs/users_guide/glasgow_exts.xml
@@ -279,7 +279,7 @@ became out of date, and wrong information is worse than none.
The Real Truth about what primitive types there are, and what operations
work over those types, is held in the file
-fptools/ghc/compiler/prelude/primops.txt.
+fptools/ghc/compiler/prelude/primops.txt.pp.
This file is used directly to generate GHC's primitive-operation definitions, so
it is always correct! It is also intended for processing into text.
@@ -343,11 +343,16 @@ it is accidental that it is represented by a pointer. If a pointer
represents a primitive value, then it really does point to that value:
no unevaluated thunks, no indirections…nothing can be at the
other end of the pointer than the primitive value.
+A numerically-intensive program using unboxed types can
+go a lot faster than its “standard”
+counterpart—we saw a threefold speedup on one example.
-There are some restrictions on the use of primitive types, the main
-one being that you can't pass a primitive value to a polymorphic
+There are some restrictions on the use of primitive types:
+
+The main restriction
+is that you can't pass a primitive value to a polymorphic
function or store one in a polymorphic data type. This rules out
things like [Int#] (i.e. lists of primitive
integers). The reason for this restriction is that polymorphic
@@ -359,11 +364,33 @@ attempt to dereference the pointer, with disastrous results. Even
worse, the unboxed value might be larger than a pointer
(Double# for instance).
+
+ You cannot bind a variable with an unboxed type
+in a top-level binding.
+
+ You cannot bind a variable with an unboxed type
+in a recursive binding.
+
+ You may bind unboxed variables in a (non-recursive,
+non-top-level) pattern binding, but any such variable causes the entire
+pattern-match
+to become strict. For example:
+
+ data Foo = Foo Int Int#
-
-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.
+ f x = let (Foo a b, w) = ..rhs.. in ..body..
+
+Since b has type Int#, the entire pattern
+match
+is strict, and the program behaves as if you had written
+
+ data Foo = Foo Int Int#
+
+ f x = case ..rhs.. of { (Foo a b, w) -> ..body.. }
+
+
+
+
@@ -398,21 +425,19 @@ values, but they avoid the heap allocation normally associated with
using fully-fledged tuples. When an unboxed tuple is returned, the
components are put directly into registers or on the stack; the
unboxed tuple itself does not have a composite representation. Many
-of the primitive operations listed in this section return unboxed
+of the primitive operations listed in primops.txt.pp return unboxed
tuples.
+In particular, the IO and ST monads use unboxed
+tuples to avoid unnecessary allocation during sequences of operations.
There are some pretty stringent restrictions on the use of unboxed tuples:
-
-
-
-
- Unboxed tuple types are subject to the same restrictions as
+Values of unboxed tuple types are subject to the same restrictions as
other unboxed types; i.e. they may not be stored in polymorphic data
structures or passed to polymorphic functions.
@@ -421,56 +446,46 @@ structures or passed to polymorphic functions.
- Unboxed tuples may only be constructed as the direct result of
-a function, and may only be deconstructed with a case expression.
-eg. the following are valid:
+No variable can have an unboxed tuple type, nor may a constructor or function
+argument have an unboxed tuple type. The following are all illegal:
-f x y = (# x+1, y-1 #)
-g x = case f x x of { (# a, b #) -> a + b }
-
-
+ data Foo = Foo (# Int, Int #)
-but the following are invalid:
+ f :: (# Int, Int #) -> (# Int, Int #)
+ f x = x
+ g :: (# Int, Int #) -> Int
+ g (# a,b #) = a
-
-f x y = g (# x, y #)
-g (# x, y #) = x + y
+ h x = let y = (# x,x #) in ...
-
-
-
-
-
- No variable can have an unboxed tuple type. This is illegal:
-
-
-
-f :: (# Int, Int #) -> (# Int, Int #)
-f x = x
-
-
-
-because x has an unboxed tuple type.
-
-
-
-
-
-
-
-
-Note: we may relax some of these restrictions in the future.
-
-The IO and ST monads use unboxed
-tuples to avoid unnecessary allocation during sequences of operations.
+The typical use of unboxed tuples is simply to return multiple values,
+binding those multiple results with a case expression, thus:
+
+ f x y = (# x+1, y-1 #)
+ g x = case f x x of { (# a, b #) -> a + b }
+
+You can have an unboxed tuple in a pattern binding, thus
+
+ f x = let (# p,q #) = h x in ..body..
+
+If the types of p and q are not unboxed,
+the resulting binding is lazy like any other Haskell pattern binding. The
+above example desugars like this:
+
+ f x = let t = case h x o f{ (# p,q #) -> (p,q)
+ p = fst t
+ q = snd t
+ in ..body..
+
+Indeed, the bindings can even be recursive.
@@ -810,68 +825,68 @@ This name is not supported by GHC.
So the flag causes
the following pieces of built-in syntax to refer to
whatever is in scope, not the Prelude
- versions:
+ versions:
- Integer and fractional literals mean
- "fromInteger 1" and
- "fromRational 3.2", not the
- Prelude-qualified versions; both in expressions and in
- patterns.
- However, the standard Prelude Eq class
- is still used for the equality test necessary for literal patterns.
-
+ An integer literal 368 means
+ "fromInteger (368::Integer)", rather than
+ "Prelude.fromInteger (368::Integer)".
+
-
- Negation (e.g. "- (f x)")
- means "negate (f x)" (not
- Prelude.negate).
-
+ Fractional literals are handed in just the same way,
+ except that the translation is
+ fromRational (3.68::Rational).
+
+
+ The equality test in an overloaded numeric pattern
+ uses whatever (==) is in scope.
+
+
+ The subtraction operation, and the
+ greater-than-or-equal test, in n+k patterns
+ use whatever (-) and (>=) are in scope.
+
- 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.(-)").
-
+ Negation (e.g. "- (f x)")
+ means "negate (f x)", both in numeric
+ patterns, and expressions.
+ "Do" notation is translated using whatever
functions (>>=),
- (>>), fail, and
- return, are in scope (not the Prelude
- versions). List comprehensions, and parallel array
+ (>>), and fail,
+ are in scope (not the Prelude
+ versions). List comprehensions, mdo (), and parallel array
comprehensions, are unaffected.
- Similarly recursive do notation (see
- ) uses whatever
- mfix function is in scope, and arrow
+ Arrow
notation (see )
uses whatever arr,
(>>>), first,
app, (|||) and
- loop functions are in scope.
-
+ loop functions are in scope. But unlike the
+ other constructs, the types of these functions must match the
+ Prelude types very closely. Details are in flux; if you want
+ to use this, ask!
+
-
- The functions with these names that GHC finds in scope
- must have types matching those of the originals, namely:
-
- fromInteger :: Integer -> N
- fromRational :: Rational -> N
- negate :: N -> N
- (-) :: N -> N -> N
- (>>=) :: forall a b. M a -> (a -> M b) -> M b
- (>>) :: forall a b. M a -> M b -> M b
- return :: forall a. a -> M a
- fail :: forall a. String -> M a
-
- (Here N may be any type,
- and M any type constructor.)
-
+In all cases (apart from arrow notation), the static semantics should be that of the desugared form,
+even if that is a little unexpected. For emample, the
+static semantics of the literal 368
+is exactly that of fromInteger (368::Integer); it's fine for
+fromInteger to have any of the types:
+
+fromInteger :: Integer -> Integer
+fromInteger :: forall a. Foo a => Integer -> a
+fromInteger :: Num a => a -> Integer
+fromInteger :: Integer -> Bool -> Bool
+
+
+
Be warned: this is an experimental facility, with
fewer checks than usual. Use -dcore-lint
to typecheck the desugared program. If Core Lint is happy
@@ -910,18 +925,49 @@ Nevertheless, they can be useful when defining "phantom types".
-Infix type constructors
+Infix type constructors, classes, and type variables
-GHC allows type constructors to be operators, and to be written infix, very much
-like expressions. More specifically:
+GHC allows type constructors, classes, and type variables to be operators, and
+to be written infix, very much like expressions. More specifically:
- A type constructor can be an operator, beginning with a colon; e.g. :*:.
+ A type constructor or class can be an operator, beginning with a colon; e.g. :*:.
The lexical syntax is the same as that for data constructors.
- Types can be written infix. For example Int :*: Bool.
+ Data type and type-synonym declarations can be written infix, parenthesised
+ if you want further arguments. E.g.
+
+ data a :*: b = Foo a b
+ type a :+: b = Either a b
+ class a :=: b where ...
+
+ data (a :**: b) x = Baz a b x
+ type (a :++: b) y = Either (a,b) y
+
+
+
+ Types, and class constraints, can be written infix. For example
+
+ x :: Int :*: Bool
+ f :: (a :=: b) => a -> b
+
+
+
+ A type variable can be an (unqualified) operator e.g. +.
+ The lexical syntax is the same as that for variable operators, excluding "(.)",
+ "(!)", and "(*)". In a binding position, the operator must be
+ parenthesised. For example:
+
+ type T (+) = Int + Int
+ f :: T Either
+ f = Left 3
+
+ liftA2 :: Arrow (~>)
+ => (a -> b -> c) -> (e ~> a) -> (e ~> b) -> (e ~> c)
+ liftA2 = ...
+
Back-quotes work
@@ -929,7 +975,7 @@ like expressions. More specifically:
Int `a` Bool. Similarly, parentheses work the same; e.g. (:*:) Int Bool.
- Fixities may be declared for type constructors just as for data constructors. However,
+ Fixities may be declared for type constructors, or classes, just as for data constructors. However,
one cannot distinguish between the two in a fixity declaration; a fixity declaration
sets the fixity for a data constructor and the corresponding type constructor. For example:
@@ -942,21 +988,6 @@ like expressions. More specifically:
Function arrow is infixr with fixity 0. (This might change; I'm not sure what it should be.)
-
- Data type and type-synonym declarations can be written infix. E.g.
-
- data a :*: b = Foo a b
- type a :+: b = Either a b
-
-
-
- The only thing that differs between operators in types and operators in expressions is that
- ordinary non-constructor operators, such as + and *
- are not allowed in types. Reason: the uniform thing to do would be to make them type
- variables, but that's not very useful. A less uniform but more useful thing would be to
- allow them to be type constructors. But that gives trouble in export
- lists. So for now we just exclude them.
-
@@ -1061,8 +1092,12 @@ because GHC does not allow unboxed tuples on the left of a function arrow.
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
+was suggested by Perry, and implemented in Hope+ (Nigel Perry, The Implementation
+of Practical Functional Programming Languages, PhD Thesis, University of
+London, 1991). It was later formalised by Laufer and Odersky
+(Polymorphic type inference and abstract data types,
+TOPLAS, 16(5), pp1411-1430, 1994).
+It's been in Lennart
Augustsson's hbc Haskell compiler for several years, and
proved very useful. Here's the idea. Consider the declaration:
@@ -1174,7 +1209,7 @@ adding a new existential quantification construct.
Type classes
-An easy extension (implemented in hbc) is to allow
+An easy extension is to allow
arbitrary contexts before the constructor. For example:
@@ -1233,6 +1268,86 @@ universal quantification earlier.
+Record Constructors
+
+
+GHC allows existentials to be used with records syntax as well. For example:
+
+
+data Counter a = forall self. NewCounter
+ { _this :: self
+ , _inc :: self -> self
+ , _display :: self -> IO ()
+ , tag :: a
+ }
+
+Here tag is a public field, with a well-typed selector
+function tag :: Counter a -> a. The self
+type is hidden from the outside; any attempt to apply _this,
+_inc or _output as functions will raise a
+compile-time error. In other words, GHC defines a record selector function
+only for fields whose type does not mention the existentially-quantified variables.
+(This example used an underscore in the fields for which record selectors
+will not be defined, but that is only programming style; GHC ignores them.)
+
+
+
+To make use of these hidden fields, we need to create some helper functions:
+
+
+inc :: Counter a -> Counter a
+inc (NewCounter x i d t) = NewCounter
+ { _this = i x, _inc = i, _display = d, tag = t }
+
+display :: Counter a -> IO ()
+display NewCounter{ _this = x, _display = d } = d x
+
+
+Now we can define counters with different underlying implementations:
+
+
+counterA :: Counter String
+counterA = NewCounter
+ { _this = 0, _inc = (1+), _display = print, tag = "A" }
+
+counterB :: Counter String
+counterB = NewCounter
+ { _this = "", _inc = ('#':), _display = putStrLn, tag = "B" }
+
+main = do
+ display (inc counterA) -- prints "1"
+ display (inc (inc counterB)) -- prints "##"
+
+
+In GADT declarations (see ), the explicit
+forall may be omitted. For example, we can express
+the same Counter a using GADT:
+
+
+data Counter a where
+ NewCounter { _this :: self
+ , _inc :: self -> self
+ , _display :: self -> IO ()
+ , tag :: a
+ }
+ :: Counter a
+
+
+At the moment, record update syntax is only supported for Haskell 98 data types,
+so the following function does not work:
+
+
+-- This is invalid; use explicit NewCounter instead for now
+setTag :: Counter a -> a -> Counter a
+setTag obj t = obj{ tag = t }
+
+
+
+
+
+
+
+Restrictions
@@ -1394,19 +1509,20 @@ declarations. Define your own instances!
Class declarations
-This section documents GHC's implementation of multi-parameter type
-classes. There's lots of background in the paper Type
+This section, and the next one, documents GHC's type-class extensions.
+There's lots of background in the paper Type
classes: exploring the design space (Simon Peyton Jones, Mark
Jones, Erik Meijer).
-There are the following constraints on class declarations:
-
-
+All the extensions are enabled by the flag.
+
+
+Multi-parameter type classes
- Multi-parameter type classes are permitted. For example:
+Multi-parameter type classes are permitted. For example:
@@ -1415,39 +1531,16 @@ There are the following constraints on class declarations:
...etc.
-
-
-
-
-
-
- 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
- }
-
- 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.)
+
-
-
-
+
+The superclasses of a class declaration
- 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:
@@ -1460,62 +1553,33 @@ be acyclic. So these class declarations are OK:
-
-
-
-
- All of the class type variables must be reachable (in the sense
-mentioned in )
-from the free variables of each method type
-. For example:
+As in Haskell 98, The class hierarchy must be acyclic. However, the definition
+of "acyclic" involves only the superclass relationships. For example,
+this is OK:
- class Coll s a where
- empty :: 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
-types, and has the same motivation.
-
-Sometimes, offending class declarations exhibit misunderstandings. For
-example, Coll might be rewritten
-
+ class C a where {
+ op :: D b => a -> b -> b
+ }
-
- class Coll s a where
- empty :: s a
- insert :: s a -> a -> s a
+ class C a => D a where { ... }
-which makes the connection between the type of a collection of
-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
-
+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.)
+
+
-
-
-
-
Class method types
+
Haskell 98 prohibits class method types to mention constraints on the
class type variable, thus:
@@ -1527,187 +1591,121 @@ class type variable, thus:
The type of elem is illegal in Haskell 98, because it
contains the constraint Eq a, constrains only the
class type variable (in this case a).
+GHC lifts this restriction.
-
-With the GHC lifts this restriction.
-
+
-
-
-Type signatures
+
+Functional dependencies
+
-The context of a type signature
+ 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,
+.
+
-Unlike Haskell 98, constraints in types do not have to be of
-the form (class type-variable) or
-(class (type-variable type-variable ...)). Thus,
-these type signatures are perfectly OK
+Functional dependencies are introduced by a vertical bar in the syntax of a
+class declaration; e.g.
- g :: Eq [a] => ...
- g :: Ord (T a ()) => ...
+ class (Monad m) => MonadState s m | m -> s where ...
+
+ class Foo a b c | a b -> c where ...
+There should be more documentation, but there isn't (yet). Yell if you need it.
-GHC imposes the following restrictions on the constraints in a type signature.
-Consider the type:
+In a class declaration, all of the class type variables must be reachable (in the sense
+mentioned in )
+from the free variables of each method type.
+For example:
- forall tv1..tvn (c1, ...,cn) => type
+ class Coll s a where
+ empty :: s
+ insert :: s -> a -> s
-(Here, we write the "foralls" explicitly, although the Haskell source
-language omits them; in Haskell 98, 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 ).
-
-
-
-
-
-
-
-
- Each universally quantified type variable
-tvi must be reachable from type.
-
-A type variable a is "reachable" if it it appears
-in the same constraint as either a type variable free in in
-type, or another reachable type variable.
-A value with a type that does not obey
-this reachability restriction cannot be used without introducing
-ambiguity; that is why the type is rejected.
-Here, for example, is an illegal type:
-
-
+is not OK, because the type of empty doesn't mention
+a. Functional dependencies can make the type variable
+reachable:
- forall a. Eq a => Int
+ class Coll s a | s -> a where
+ empty :: s
+ insert :: s -> a -> s
+Alternatively Coll might be rewritten
-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.
-
-
-Note
-that the reachability condition is weaker than saying that a is
-functionally dependent on a type variable free in
-type (see ). The reason for this is there
-might be a "hidden" dependency, in a superclass perhaps. So
-"reachable" is a conservative approximation to "functionally dependent".
-For example, consider:
- class C a b | a -> b where ...
- class C a b => D a b where ...
- f :: forall a b. D a b => a -> a
+ class Coll s a where
+ empty :: s a
+ insert :: s a -> a -> s a
-This is fine, because in fact a does functionally determine b
-but that is not immediately apparent from f's type.
-
-
-
-
- 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:
+which makes the connection between the type of a collection of
+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:
- forall a. C a b => burble
-
-
+ class CollE s where
+ empty :: s
-The next type is illegal because the constraint Eq b does not
-mention a:
+ class CollE s => Coll s a where
+ insert :: s -> a -> s
+
+
+
-
- forall a. Eq b => burble
-
-The reason for this restriction is milder than the other one. The
-excluded types are never useful or necessary (because the offending
-context doesn't need to be witnessed at this point; it can be floated
-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.
-
-
+
-
+
+Instance declarations
-
-
+
+Instance heads
-
-For-all hoisting
-It is often convenient to use generalised type synonyms (see ) 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 a1..an. context2 => type2
-==>
- forall a1..an. context2 => 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
-
+The head of an instance declaration is the part to the
+right of the "=>". In Haskell 98 the head of an instance
+declaration
+must be of the form C (T a1 ... an), where
+C is the class, T is a type constructor,
+and the a1 ... an are distinct type variables.
-When doing this hoisting operation, GHC eliminates duplicate constraints. For
-example:
-
- type Foo a = (?x::Int) => Bool -> a
- g :: Foo (Foo Int)
-
-means
+The flag lifts this restriction and allows the
+instance head to be of form C t1 ... tn where t1
+... tn are arbitrary types (provided, of course, everything is
+well-kinded). In particular, types ti can be type variables
+or structured types, and can contain repeated occurrences of a single type
+variable.
+Examples:
- g :: (?x::Int) => Bool -> Bool -> Int
+ instance Eq (T a a) where ...
+ -- Repeated type variable
+
+ instance Eq (S [a]) where ...
+ -- Structured type
+
+ instance C Int [a] where ...
+ -- Multiple parameters
-
-
-
-
-Instance declarations
-
-
+Overlapping instances
In general, GHC requires that that it be unambiguous which instance
@@ -1731,7 +1729,8 @@ these declarations:
instance context3 => C Int [a] where ... -- (C)
instance context4 => C Int [Int] where ... -- (D)
-The instances (A) and (B) match the constraint C Int Bool, but (C) and (D) do not. When matching, GHC takes
+The instances (A) and (B) match the constraint C Int Bool,
+but (C) and (D) do not. When matching, GHC takes
no account of the context of the instance declaration
(context1 etc).
GHC's default behaviour is that exactly one instance must match the
@@ -1763,6 +1762,35 @@ So GHC rejects the program. If you add the flag
+
+The willingness to be overlapped or incoherent is a property of
+the instance declaration itself, controlled by the
+presence or otherwise of the
+and flags when that mdodule is
+being defined. Neither flag is required in a module that imports and uses the
+instance declaration. Specifically, during the lookup process:
+
+
+An instance declaration is ignored during the lookup process if (a) a more specific
+match is found, and (b) the instance declaration was compiled with
+. The flag setting for the
+more-specific instance does not matter.
+
+
+Suppose an instance declaration does not matche the constraint being looked up, but
+does unify with it, so that it might match when the constraint is further
+instantiated. Usually GHC will regard this as a reason for not committing to
+some other constraint. But if the instance declaration was compiled with
+, GHC will skip the "does-it-unify?"
+check for that declaration.
+
+
+All this makes it possible for a library author to design a library that relies on
+overlapping instances without the library client having to know.
+
+The flag implies the
+ flag, but not vice versa.
+
@@ -1827,7 +1855,7 @@ but this is not:
instance F 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 ...
@@ -1838,78 +1866,251 @@ For example, this is OK:
All of the types in the context of
-an instance declaration must be type variables.
+an instance declaration must be type variables, and
+there must be no repeated type variables in any one constraint.
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 ...
+
+is not OK (because of the non-type-variable Int in the context), and nor is
+
+instance C b b => Foo [b] where ...
+
+(because of the repeated type variable).
+
+
+
+These restrictions ensure that
+context reduction terminates: each reduction step removes one type
+constructor. For example, the following would make the type checker
+loop if it wasn't excluded:
+
+ instance C a => C a where ...
+
+There are two situations in which the rule is a bit of a pain. First,
+if one allows overlapping instance declarations then it's quite
+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 { }
+
+ instance (C1 a, C2 a, C3 a) => C a where { }
+
+
+
+This allows you to write shorter signatures:
+
+
+
+ f :: C a => ...
+
+
+
+instead of
+
+
+
+ f :: (C1 a, C2 a, C3 a) => ...
+
+
+
+Voluminous correspondence on the Haskell mailing list has convinced me
+that it's worth experimenting with more liberal rules. If you use
+the experimental flag
+-fallow-undecidable-instances
+option, you can use arbitrary
+types in both an instance context and instance head. 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 N.
+
+
+I'm on the lookout for a less brutal solution: a simple rule that preserves decidability while
+allowing these idioms interesting idioms.
+
+
+
+
+
+
+
+Type signatures
+
+The context of a type signature
+
+Unlike Haskell 98, constraints in types do not have to be of
+the form (class type-variable) or
+(class (type-variable type-variable ...)). Thus,
+these type signatures are perfectly OK
+
+ g :: Eq [a] => ...
+ g :: Ord (T a ()) => ...
+
+
+
+GHC imposes the following restrictions on the constraints in a type signature.
+Consider the type:
+
+
+ forall tv1..tvn (c1, ...,cn) => type
+
+
+(Here, we write the "foralls" explicitly, although the Haskell source
+language omits them; in Haskell 98, 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 ).
+
+
+
+
+
+
+
+
+ Each universally quantified type variable
+tvi must be reachable from type.
+
+A type variable a is "reachable" if it it appears
+in the same constraint as either a type variable free in in
+type, or another reachable type variable.
+A value with a type that does not obey
+this reachability restriction cannot be used without introducing
+ambiguity; that is why the type is rejected.
+Here, for example, is an illegal type:
+
+
+
+ forall a. Eq a => Int
-is OK, but
+
+
+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.
+
+
+Note
+that the reachability condition is weaker than saying that a is
+functionally dependent on a type variable free in
+type (see ). The reason for this is there
+might be a "hidden" dependency, in a superclass perhaps. So
+"reachable" is a conservative approximation to "functionally dependent".
+For example, consider:
-instance C Int b => Foo b where ...
+ class C a b | a -> b where ...
+ class C a b => D a b where ...
+ f :: forall a b. D a b => a -> a
-is not OK.
+This is fine, because in fact a does functionally determine b
+but that is not immediately apparent from f's type.
-
-These restrictions ensure that
-context reduction terminates: each reduction step removes one type
-constructor. For example, the following would make the type checker
-loop if it wasn't excluded:
-
- instance C a => C a where ...
-
-There are two situations in which the rule is a bit of a pain. First,
-if one allows overlapping instance declarations then it's quite
-convenient to have a "default instance" declaration that applies if
-something more specific does not:
+
+
+
+ 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:
- instance C a where
- op = ... -- Default
+ forall a. C a b => burble
-Second, sometimes you might want to use the following to get the
-effect of a "class synonym":
+The next type is illegal because the constraint Eq b does not
+mention a:
- class (C1 a, C2 a, C3 a) => C a where { }
-
- instance (C1 a, C2 a, C3 a) => C a where { }
+ forall a. Eq b => burble
-This allows you to write shorter signatures:
-
+The reason for this restriction is milder than the other one. The
+excluded types are never useful or necessary (because the offending
+context doesn't need to be witnessed at this point; it can be floated
+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.
-
- f :: C a => ...
-
+
+
+
-instead of
+
+
+
+For-all hoisting
+
+It is often convenient to use generalised type synonyms (see ) 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
- f :: (C1 a, C2 a, C3 a) => ...
+ 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 a1..an. context2 => type2
+==>
+ forall a1..an. context2 => 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
-
-
-Voluminous correspondence on the Haskell mailing list has convinced me
-that it's worth experimenting with more liberal rules. If you use
-the experimental flag
--fallow-undecidable-instances
-option, you can use arbitrary
-types in both an instance context and instance head. 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 N.
-I'm on the lookout for a less brutal solution: a simple rule that preserves decidability while
-allowing these idioms interesting idioms.
+When doing this hoisting operation, GHC eliminates duplicate constraints. For
+example:
+
+ type Foo a = (?x::Int) => Bool -> a
+ g :: Foo (Foo Int)
+
+means
+
+ g :: (?x::Int) => Bool -> Bool -> Int
+
@@ -2079,6 +2280,68 @@ the binding for ?x, so the type of f is
+
+Implicit parameters and polymorphic recursion
+
+
+Consider these two definitions:
+
+ len1 :: [a] -> Int
+ len1 xs = let ?acc = 0 in len_acc1 xs
+
+ len_acc1 [] = ?acc
+ len_acc1 (x:xs) = let ?acc = ?acc + (1::Int) in len_acc1 xs
+
+ ------------
+
+ len2 :: [a] -> Int
+ len2 xs = let ?acc = 0 in len_acc2 xs
+
+ len_acc2 :: (?acc :: Int) => [a] -> Int
+ len_acc2 [] = ?acc
+ len_acc2 (x:xs) = let ?acc = ?acc + (1::Int) in len_acc2 xs
+
+The only difference between the two groups is that in the second group
+len_acc is given a type signature.
+In the former case, len_acc1 is monomorphic in its own
+right-hand side, so the implicit parameter ?acc is not
+passed to the recursive call. In the latter case, because len_acc2
+has a type signature, the recursive call is made to the
+polymoprhic version, which takes ?acc
+as an implicit parameter. So we get the following results in GHCi:
+
+ Prog> len1 "hello"
+ 0
+ Prog> len2 "hello"
+ 5
+
+Adding a type signature dramatically changes the result! This is a rather
+counter-intuitive phenomenon, worth watching out for.
+
+
+
+Implicit parameters and monomorphism
+
+GHC applies the dreaded Monomorphism Restriction (section 4.5.5 of the
+Haskell Report) to implicit parameters. For example, consider:
+
+ f :: Int -> Int
+ f v = let ?x = 0 in
+ let y = ?x + v in
+ let ?x = 5 in
+ y
+
+Since the binding for y falls under the Monomorphism
+Restriction it is not generalised, so the type of y is
+simply Int, not (?x::Int) => Int.
+Hence, (f 9) returns result 9.
+If you add a type signature for y, then y
+will get type (?x::Int) => Int, so the occurrence of
+y in the body of the let will see the
+inner binding of ?x, so (f 9) will return
+14.
+
+
@@ -2251,30 +2514,6 @@ and you'd be right. That is why they are an experimental feature.
-
-Functional dependencies
-
-
- 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,
-.
-
-
-Functional dependencies are introduced by a vertical bar in the syntax of a
-class declaration; e.g.
-
- class (Monad m) => MonadState s m | m -> s where ...
-
- class Foo a b c | a b -> c where ...
-
-There should be more documentation, but there isn't (yet). Yell if you need it.
-
-
-
-
-
Explicitly-kinded quantification
@@ -2649,22 +2888,19 @@ for rank-2 types.
-A pattern type signature can introduce a scoped type
-variable. For example
-
-
-
-
+A lexically scoped type variable can be bound by:
+
+A declaration type signature ()
+A pattern type signature ()
+A result type signature ()
+
+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.
@@ -2672,8 +2908,6 @@ In particular, it is in scope at the type signature for y.
- 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
@@ -2696,10 +2930,10 @@ So much for the basic idea. Here are the details.
-What a pattern type signature means
+What a scoped type variable 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
+A lexically-scoped type variable is simply
+the name for a type. The restriction it expresses is that all occurrences
of the same name mean the same type. For example:
f :: [Int] -> Int -> Int
@@ -2869,7 +3103,33 @@ scope over the methods defined in the where part. For exampl
-
+
+Declaration type signatures
+A declaration type signature that has explicit
+quantification (using forall) brings into scope the
+explicitly-quantified
+type variables, in the definition of the named function(s). For example:
+
+ f :: forall a. [a] -> [a]
+ f (x:xs) = xs ++ [ x :: a ]
+
+The "forall a" brings "a" into scope in
+the definition of "f".
+
+This only happens if the quantification in f's type
+signature is explicit. For example:
+
+ g :: [a] -> [a]
+ g (x:xs) = xs ++ [ x :: a ]
+
+This program will be rejected, because "a" does not scope
+over the definition of "f", so "x::a"
+means "x::forall a. a" by Haskell's usual implicit
+quantification rules.
+
+
+
+Where a pattern type signature can occur
@@ -2986,10 +3246,12 @@ in f4's scope.
+Pattern type signatures are completely orthogonal to ordinary, separate
+type signatures. The two can be used independently or together.
-
+Result type signatures
@@ -3059,9 +3321,23 @@ classes Eq, Ord,
GHC extends this list with two more classes that may be automatically derived
(provided the flag is specified):
Typeable, and Data. These classes are defined in the library
-modules Data.Dynamic and Data.Generics respectively, and the
+modules Data.Typeable and Data.Generics respectively, and the
appropriate class must be in scope before it can be mentioned in the deriving clause.
+An instance of Typeable can only be derived if the
+data type has seven or fewer type parameters, all of kind *.
+The reason for this is that the Typeable class is derived using the scheme
+described in
+
+Scrap More Boilerplate: Reflection, Zips, and Generalised Casts
+.
+(Section 7.4 of the paper describes the multiple Typeable classes that
+are used, and only Typeable1 up to
+Typeable7 are provided in the library.)
+In other cases, there is nothing to stop the programmer writing a TypableX
+class, whose kind suits that of the data type constructor, and
+then writing the data type instance by hand.
+
@@ -3174,7 +3450,7 @@ class to the new type:
As a result of this extension, all derived instances in newtype
-declarations are treated uniformly (and implemented just by reusing
+ declarations are treated uniformly (and implemented just by reusing
the dictionary for the representation type), exceptShow and Read, which really behave differently for
the newtype and its representation.
@@ -3255,10 +3531,80 @@ then we would not have been able to derive an instance for the
classes usually have one "main" parameter for which deriving new
instances is most interesting.
+Lastly, all of this applies only for classes other than
+Read, Show, Typeable,
+and Data, for which the built-in derivation applies (section
+4.3.3. of the Haskell Report).
+(For the standard classes Eq, Ord,
+Ix, and Bounded it is immaterial whether
+the standard method is used or the one described here.)
+
+
+Generalised typing of mutually recursive bindings
+
+
+The Haskell Report specifies that a group of bindings (at top level, or in a
+let or where) should be sorted into
+strongly-connected components, and then type-checked in dependency order
+(Haskell
+Report, Section 4.5.1).
+As each group is type-checked, any binders of the group that
+have
+an explicit type signature are put in the type environment with the specified
+polymorphic type,
+and all others are monomorphic until the group is generalised
+(Haskell Report, Section 4.5.2).
+
+
+Following a suggestion of Mark Jones, in his paper
+Typing Haskell in
+Haskell,
+GHC implements a more general scheme. If is
+specified:
+the dependency analysis ignores references to variables that have an explicit
+type signature.
+As a result of this refined dependency analysis, the dependency groups are smaller, and more bindings will
+typecheck. For example, consider:
+
+ f :: Eq a => a -> Bool
+ f x = (x == x) || g True || g "Yes"
+
+ g y = (y <= y) || f True
+
+This is rejected by Haskell 98, but under Jones's scheme the definition for
+g is typechecked first, separately from that for
+f,
+because the reference to f in g's right
+hand side is ingored by the dependency analysis. Then g's
+type is generalised, to get
+
+ g :: Ord a => a -> Bool
+
+Now, the defintion for f is typechecked, with this type for
+g in the type environment.
+
+
+
+The same refined dependency analysis also allows the type signatures of
+mutually-recursive functions to have different contexts, something that is illegal in
+Haskell 98 (Section 4.5.2, last sentence). With
+
+GHC only insists that the type signatures of a refined group have identical
+type signatures; in practice this means that only variables bound by the same
+pattern binding must have the same context. For example, this is fine:
+
+ f :: Eq a => a -> Bool
+ f x = (x == x) || g True
+
+ g :: Ord a => a -> Bool
+ g y = (y <= y) || f True
+
+
+
@@ -3285,9 +3631,9 @@ for these Terms:
eval :: Term a -> a
eval (Lit i) = i
eval (Succ t) = 1 + eval t
- eval (IsZero i) = eval i == 0
+ eval (IsZero t) = eval t == 0
eval (If b e1 e2) = if eval b then eval e1 else eval e2
- eval (Pair e1 e2) = (eval e2, eval e2)
+ eval (Pair e1 e2) = (eval e1, eval e2)
These and many other examples are given in papers by Hongwei Xi, and Tim Sheard.
@@ -3319,9 +3665,55 @@ type above, the type of each constructor must end with ... -> Term ...
-You cannot use a deriving clause on a GADT-style data type declaration,
-nor can you use record syntax. (It's not clear what these constructs would mean. For example,
-the record selectors might ill-typed.) However, you can use strictness annotations, in the obvious places
+You can use record syntax on a GADT-style data type declaration:
+
+
+ data Term a where
+ Lit { val :: Int } :: Term Int
+ Succ { num :: Term Int } :: Term Int
+ Pred { num :: Term Int } :: Term Int
+ IsZero { arg :: Term Int } :: Term Bool
+ Pair { arg1 :: Term a
+ , arg2 :: Term b
+ } :: Term (a,b)
+ If { cnd :: Term Bool
+ , tru :: Term a
+ , fls :: Term a
+ } :: Term a
+
+For every constructor that has a field f, (a) the type of
+field f must be the same; and (b) the
+result type of the constructor must be the same; both modulo alpha conversion.
+Hence, in our example, we cannot merge the num and arg
+fields above into a
+single name. Although their field types are both Term Int,
+their selector functions actually have different types:
+
+
+ num :: Term Int -> Term Int
+ arg :: Term Bool -> Term Int
+
+
+At the moment, record updates are not yet possible with GADT, so support is
+limited to record construction, selection and pattern matching:
+
+
+ someTerm :: Term Bool
+ someTerm = IsZero { arg = Succ { num = Lit { val = 0 } } }
+
+ eval :: Term a -> a
+ eval Lit { val = i } = i
+ eval Succ { num = t } = eval t + 1
+ eval Pred { num = t } = eval t - 1
+ eval IsZero { arg = t } = eval t == 0
+ eval Pair { arg1 = t1, arg2 = t2 } = (eval t1, eval t2)
+ eval t@If{} = if eval (cnd t) then eval (tru t) else eval (fls t)
+
+
+
+
+
+You can use strictness annotations, in the obvious places
in the constructor type:
data Term a where
@@ -3332,6 +3724,23 @@ in the constructor type:
+You can use a deriving clause on a GADT-style data type
+declaration, but only if the data type could also have been declared in
+Haskell-98 syntax. For example, these two declarations are equivalent
+
+ data Maybe1 a where {
+ Nothing1 :: Maybe a ;
+ Just1 :: a -> Maybe a
+ } deriving( Eq, Ord )
+
+ data Maybe2 a = Nothing2 | Just2 a
+ deriving( Eq, Ord )
+
+This simply allows you to declare a vanilla Haskell-98 data type using the
+where form without losing the deriving clause.
+
+
+
Pattern matching causes type refinement. For example, in the right hand side of the equation
eval :: Term a -> a
@@ -3359,7 +3768,7 @@ the result type of the case expression. Hence the addition <
Notice that GADTs generalise existential types. For example, these two declarations are equivalent:
data T a = forall b. MkT b (b->a)
- data T' a where { MKT :: b -> (b->a) -> T a }
+ data T' a where { MKT :: b -> (b->a) -> T' a }
@@ -3415,9 +3824,11 @@ Tim Sheard is going to expand it.)
A splice can occur in place of
- an expression; the spliced expression must have type Expr
+ an expression; the spliced expression must
+ have type Q Exp a list of top-level declarations; ; the spliced expression must have type Q [Dec]
- a type; the spliced expression must have type Type.
+ [Planned, but not implemented yet.] a
+ type; the spliced expression must have type Q Typ.
(Note that the syntax for a declaration splice uses "$" not "splice" as in
the paper. Also the type of the enclosed expression must be Q [Dec], not [Q Dec]
@@ -3432,7 +3843,7 @@ Tim Sheard is going to expand it.)
the quotation has type Expr.[d| ... |], where the "..." is a list of top-level declarations;
the quotation has type Q [Dec].
- [t| ... |], where the "..." is a type;
+ [Planned, but not implemented yet.] [t| ... |], where the "..." is a type;
the quotation has type Type.
@@ -3594,7 +4005,7 @@ What follows is a brief introduction to the notation;
it won't make much sense unless you've read Hughes's paper.
This notation is translated to ordinary Haskell,
using combinators from the
-Control.Arrow
+Control.Arrow
module.
@@ -3678,7 +4089,7 @@ proc x -> f x -<< x+1
which is equivalent to
-arr (\ x -> (f, x+1)) >>> app
+arr (\ x -> (f x, x+1)) >>> app
so in this case the arrow must belong to the ArrowApply
class.
@@ -3707,7 +4118,7 @@ the arrow f, and matches its output against
y.
In the next line, the output is discarded.
The arrow returnA is defined in the
-Control.Arrow
+Control.Arrow
module as arr id.
The above example is treated as an abbreviation for
@@ -3724,7 +4135,7 @@ arr (\ x -> (x, x)) >>>
Note that variables not used later in the composition are projected out.
After simplification using rewrite rules (see )
defined in the
-Control.Arrow
+Control.Arrow
module, this reduces to
arr (\ x -> (x+1, x)) >>>
@@ -4019,7 +4430,7 @@ additional restrictions:
The module must import
-Control.Arrow.
+Control.Arrow.
@@ -4115,12 +4526,13 @@ can still define and use your own versions of
-To have the compiler ignore uses of assert, use the compiler option
-. -fignore-asserts
-option That is, expressions of the form
+GHC ignores assertions when optimisation is turned on with the
+ flag. That is, expressions of the form
assert pred e will be rewritten to
-e.
-
+e. You can also disable assertions using the
+
+ option
+ .
Assertion failures can be caught, see the documentation for the
@@ -4177,7 +4589,7 @@ Assertion failures can be caught, see the documentation for the
- You can deprecate a function, class, or type, with the
+ You can deprecate a function, class, type, or data constructor, with the
following top-level declaration:
{-# DEPRECATED f, C, T "Don't use these" #-}
@@ -4185,6 +4597,13 @@ Assertion failures can be caught, see the documentation for the
When you compile any module that imports and uses any
of the specified entities, GHC will print the specified
message.
+ You can only depecate entities declared at top level in the module
+ being compiled, and you can only use unqualified names in the list of
+ entities being deprecated. A capitalised name, such as T
+ refers to either the type constructor T
+ or the data constructor T, or both if
+ both are in scope. If both are in scope, there is currently no way to deprecate
+ one without the other (c.f. fixities ).
Any use of the deprecated item, or of anything from a deprecated
@@ -4200,6 +4619,31 @@ Assertion failures can be caught, see the documentation for the
.
+
+ INCLUDE pragma
+
+ The INCLUDE pragma is for specifying the names
+ of C header files that should be #include'd into
+ the C source code generated by the compiler for the current module (if
+ compiling via C). For example:
+
+
+{-# INCLUDE "foo.h" #-}
+{-# INCLUDE <stdio.h> #-}
+
+ The INCLUDE pragma(s) must appear at the top of
+ your source file with any OPTIONS_GHC
+ pragma(s).
+
+ An INCLUDE pragma is the preferred alternative
+ to the option (), because the
+ INCLUDE pragma is understood by other
+ compilers. Yet another alternative is to add the include file to each
+ foreign import declaration in your code, but we
+ don't recommend using this approach with GHC.
+
+
INLINE and NOINLINE pragmas
@@ -4351,6 +4795,29 @@ key_function :: Int -> String -> (Bool, Double)
+
+ LANGUAGE pragma
+
+ LANGUAGEpragma
+ pragmaLANGUAGE
+
+ This allows language extensions to be enabled in a portable way.
+ It is the intention that all Haskell compilers support the
+ LANGUAGE pragma with the same syntax, although not
+ all extensions are supported by all compilers, of
+ course. The LANGUAGE pragma should be used instead
+ of OPTIONS_GHC, if possible.
+
+ For example, to enable the FFI and preprocessing with CPP:
+
+{-# LANGUAGE ForeignFunctionInterface, CPP #-}
+
+ Any extension from the Extension type defined in
+ Language.Haskell.Extension may be used. GHC will report an error if any of the requested extensions are not supported.
+
+
+
LINE pragma
@@ -4361,9 +4828,7 @@ key_function :: Int -> String -> (Bool, Double)
code. It lets you specify the line number and filename of the
original code; for example
-
-{-# LINE 42 "Foo.vhs" #-}
-
+{-# LINE 42 "Foo.vhs" #-}if you'd generated the current file from something called
Foo.vhs and this line corresponds to line
@@ -4408,7 +4873,7 @@ key_function :: Int -> String -> (Bool, Double)
overloaded function:
-hammeredLookup :: Ord key => [(key, value)] -> key -> value
+ hammeredLookup :: Ord key => [(key, value)] -> key -> value
If it is heavily used on lists with
@@ -4416,7 +4881,7 @@ hammeredLookup :: Ord key => [(key, value)] -> key -> value
follows:
-{-# SPECIALIZE hammeredLookup :: [(Widget, value)] -> Widget -> value #-}
+ {-# SPECIALIZE hammeredLookup :: [(Widget, value)] -> Widget -> value #-}
A SPECIALIZE pragma for a function can
@@ -4427,7 +4892,35 @@ hammeredLookup :: Ord key => [(key, value)] -> key -> value
(see ) that rewrites a call to the
un-specialised function into a call to the specialised one.
- In earlier versions of GHC, it was possible to provide your own
+ The type in a SPECIALIZE pragma can be any type that is less
+ polymorphic than the type of the original function. In concrete terms,
+ if the original function is f then the pragma
+
+ {-# SPECIALIZE f :: <type> #-}
+
+ is valid if and only if the defintion
+
+ f_spec :: <type>
+ f_spec = f
+
+ is valid. Here are some examples (where we only give the type signature
+ for the original function, not its code):
+
+ f :: Eq a => a -> b -> b
+ {-# SPECIALISE g :: Int -> b -> b #-}
+
+ g :: (Eq a, Ix b) => a -> b -> b
+ {-# SPECIALISE g :: (Eq a) => a -> Int -> Int #-}
+
+ h :: Eq a => a -> a -> a
+ {-# SPECIALISE h :: (Eq a) => [a] -> [a] -> [a] #-}
+
+The last of these examples will generate a
+RULE with a somewhat-complex left-hand side (try it yourself), so it might not fire very
+well. If you use this kind of specialisation, let us know how well it works.
+
+
+ Note: In earlier versions of GHC, it was possible to provide your own
specialised function for a given type: