X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=ghc%2Fdocs%2Fusers_guide%2Fglasgow_exts.sgml;h=38aa5b138405904c84e9d1ace4984711fcb5e46a;hb=bcfa0eea78dac7114c4d59f95a3f16ca78c3faf4;hp=063527bc7d9a183b9d7086ce07f3aa8859bab6a0;hpb=b866a9bcc1ec7fa6c5d26410eb4c0e49e08795c9;p=ghc-hetmet.git
diff --git a/ghc/docs/users_guide/glasgow_exts.sgml b/ghc/docs/users_guide/glasgow_exts.sgml
index 063527b..38aa5b1 100644
--- a/ghc/docs/users_guide/glasgow_exts.sgml
+++ b/ghc/docs/users_guide/glasgow_exts.sgml
@@ -110,6 +110,15 @@ with GHC.
+
+
+
+ See . Independent of
+ .
+
+
+
+
@@ -635,32 +644,6 @@ This name is not supported by GHC.
- Infix type constructors
-
-GHC supports infix type constructors, much as it supports infix data constructors. For example:
-
- infixl 5 :+:
-
- data a :+: b = Inl a | Inr b
-
- f :: a `Either` b -> a :+: b
- f (Left x) = Inl x
-
-
-The lexical
-syntax of an infix type constructor is just like that of an infix data constructor: either
-it's an operator beginning with ":", or it is an ordinary (alphabetic) type constructor enclosed in
-back-quotes.
-
-
-When you give a fixity declaration, the fixity applies to both the data constructor and the
-type constructor with the specified name. You cannot give different fixities to the type constructor T
-and the data constructor T.
-
-
-
-
-
@@ -959,38 +942,19 @@ 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
-There are the following restrictions on the form of a qualified
-type:
-
-
-
+GHC imposes the following restrictions on the form of a qualified
+type, whether declared in a type signature
+or inferred. Consider the 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,
@@ -1005,11 +969,15 @@ in GHC, you can give the foralls if you want. See )
+on the type variables 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:
+ambiguity.
+Here, for example, is an illegal type:
@@ -1064,10 +1032,6 @@ 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
@@ -1154,56 +1118,14 @@ be acyclic. So these class declarations are OK:
-
-
-
- In the signature of a class operation, every constraint
-must mention at least one type variable that is not a class type
-variable.
-
-Thus:
-
-
-
- class Collection c a where
- 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:
-
-
-
- class C a where
- 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
-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)
-
-
-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:
+ All of the class type variables must be reachable (in the sense
+mentioned in )
+from the free varibles of each method type
+. For example:
@@ -1379,63 +1301,8 @@ 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
-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) => ...
-
-
-
-I'm on the lookout for a simple rule that preserves decidability while
-allowing these idioms. The experimental flag
--fallow-undecidable-instances
-option lifts this restriction, allowing all the types in an
-instance head to be type variables.
-
+See for an experimental
+extension to lift this restriction.
@@ -1497,16 +1364,10 @@ instance C Int b => Foo b where ...
-is not OK. Again, the intent here is to make sure that context
-reduction terminates.
+is not OK. See for an experimental
+extension to lift this restriction.
+
-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 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 N.
@@ -1519,6 +1380,80 @@ with N.
+
+Undecidable instances
+
+The rules for instance declarations state that:
+
+At least one of the types in the head of
+an instance declaration must not be a type variable.
+
+All of the types in the context of
+an instance declaration must be type variables.
+
+
+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.
+
+
+
Implicit parameters
@@ -1555,7 +1490,7 @@ implicitly parameterized by a comparison function named cmp.
The dynamic binding constraints are just a new form of predicate in the type class system.
-An implicit parameter occurs in an exprssion using the special form ?x,
+An implicit parameter occurs in an expression using the special form ?x,
where x is
any valid identifier (e.g. ord ?x is a valid expression).
Use of this construct also introduces a new
@@ -1608,6 +1543,19 @@ 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.
+
+Implicit-parameter constraints do not cause ambiguity. For example, consider:
+
+ f :: (?x :: [a]) => Int -> Int
+ f n = n + length ?x
+
+ g :: (Read a, Show a) => String -> String
+ g s = show (read s)
+
+Here, g has an ambiguous type, and is rejected, but f
+is fine. The binding for ?x at f's call site is
+quite unambiguous, and fixes the type a.
+
@@ -1847,8 +1795,14 @@ 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.
@@ -2857,49 +2811,6 @@ scope over the methods defined in the where part. For exampl
-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
-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)
-
-
-
-
-
-
-
-
-
-
-
-Result type signatures are not yet implemented in Hugs.
-
-
-
-
-Where a pattern type signature can occur
@@ -3012,41 +2923,96 @@ in f4's scope.
-
-
-Generalised derived instances for newtypes
+
+Result type signatures
-When you define an abstract type using newtype, you may want
-the new type to inherit some instances from its representation. In
-Haskell 98, you can inherit instances of Eq, Ord,
-Enum and Bounded by deriving them, but for any
-other classes you have to write an explicit instance declaration. For
-example, if you define
-
-
- newtype Dollars = Dollars Int
-
+The result type of a function can be given a signature, thus:
-and you want to use arithmetic on Dollars, you have to
-explicitly define an instance of Num:
-
- instance Num Dollars where
- Dollars a + Dollars b = Dollars (a+b)
- ...
+
+ f (x::a) :: [a] = [x,x,x]
-All the instance does is apply and remove the newtype
-constructor. It is particularly galling that, since the constructor
-doesn't appear at run-time, this instance declaration defines a
-dictionary which is wholly equivalent to the Int
-dictionary, only slower!
-
- Generalising the deriving clause
-
+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)
+
+
+
+
+The type variables bound in a result type signature scope over the right hand side
+of the definition. However, consider this corner-case:
+
+ rev1 :: [a] -> [a] = \xs -> reverse xs
+
+ foo ys = rev (ys::[a])
+
+The signature on rev1 is considered a pattern type signature, not a result
+type signature, and the type variables it binds have the same scope as rev1
+itself (i.e. the right-hand side of rev1 and the rest of the module too).
+In particular, the expression (ys::[a]) is OK, because the type variable a
+is in scope (otherwise it would mean (ys::forall a.[a]), which would be rejected).
+
+
+As mentioned above, rev1 is made monomorphic by this scoping rule.
+For example, the following program would be rejected, because it claims that rev1
+is polymorphic:
+
+ rev1 :: [b] -> [b]
+ rev1 :: [a] -> [a] = \xs -> reverse xs
+
+
+
+
+Result type signatures are not yet implemented in Hugs.
+
+
+
+
+
+
+
+Generalised derived instances for newtypes
+
+
+When you define an abstract type using newtype, you may want
+the new type to inherit some instances from its representation. In
+Haskell 98, you can inherit instances of Eq, Ord,
+Enum and Bounded by deriving them, but for any
+other classes you have to write an explicit instance declaration. For
+example, if you define
+
+
+ newtype Dollars = Dollars Int
+
+
+and you want to use arithmetic on Dollars, you have to
+explicitly define an instance of Num:
+
+
+ instance Num Dollars where
+ Dollars a + Dollars b = Dollars (a+b)
+ ...
+
+All the instance does is apply and remove the newtype
+constructor. It is particularly galling that, since the constructor
+doesn't appear at run-time, this instance declaration defines a
+dictionary which is wholly equivalent to the Int
+dictionary, only slower!
+
+
+
+ Generalising the deriving clause
+
GHC now permits such instances to be derived instead, so one can write
newtype Dollars = Dollars Int deriving (Eq,Show,Num)
@@ -3140,13 +3106,26 @@ declaration (after expansion of any type synonyms)
newtype T v1...vn = T' (S t1...tk vk+1...vn) deriving (c1...cm)
-where S is a type constructor, t1...tk are
-types,
-vk+1...vn are type variables which do not occur in any of
-the ti, and the ci are partial applications of
-classes of the form C t1'...tj'. The derived instance
-declarations are, for each ci,
-
+where
+
+
+ S is a type constructor,
+
+
+ t1...tk are types,
+
+
+ vk+1...vn are type variables which do not occur in any of
+ the ti, and
+
+
+ the ci are partial applications of
+ classes of the form C t1'...tj', where the arity of C
+ is exactly j+1. That is, C lacks exactly one type argument.
+
+
+Then, for each ci, the derived instance
+declaration is:
instance ci (S t1...tk vk+1...v) => ci (T v1...vp)
@@ -3205,6 +3184,9 @@ Template Meta-programming for Haskell", in
Proc Haskell Workshop 2002.
+ The first example from that paper is set out below as a worked example to help get you started.
+
+
The documentation here describes the realisation in GHC. (It's rather sketchy just now;
Tim Sheard is going to expand it.)
@@ -3225,21 +3207,25 @@ Tim Sheard is going to expand it.)
A splice can occur in place of
- an expression;
- a list of top-level declarations;
- a pattern;
- a type;
+ an expression; the spliced expression must have type Expr
+ a list of top-level declarations; ; the spliced expression must have type Q [Dec]
+ a type; the spliced expression must have type Type.
+ (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]
+ as in the paper.)
A expression quotation is written in Oxford brackets, thus:
- [| ... |], where the "..." is an expression;
- [d| ... |], where the "..." is a list of top-level declarations;
- [p| ... |], where the "..." is a pattern;
- [t| ... |], where the "..." is a type;
+ [| ... |], where the "..." is an expression;
+ 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;
+ the quotation has type Type.
@@ -3268,12 +3254,6 @@ Tim Sheard is going to expand it.)
- If the module contains any top-level splices that must be run, you must use GHC with
- --make or --interactive flags. (Reason: that
- means it walks the dependency tree and knows what modules must be linked etc.)
-
-
-
You can only run a function at compile time if it is imported from another module. That is,
you can't define a function in a module, and call it from within a splice in the same module.
(It would make sense to do so, but it's hard to implement.)
@@ -3282,12 +3262,541 @@ Tim Sheard is going to expand it.)
The flag -ddump-splices shows the expansion of all top-level splices as they happen.
+
+ If you are building GHC from source, you need at least a stage-2 bootstrap compiler to
+ run Template Haskell. A stage-1 compiler will reject the TH constructs. Reason: TH
+ compiles and runs a program, and then looks at the result. So it's important that
+ the program it compiles produces results whose representations are identical to
+ those of the compiler itself.
+
+ Template Haskell works in any mode (--make, --interactive,
+ or file-at-a-time). There used to be a restriction to the former two, but that restriction
+ has been lifted.
+
+
+
+ A Template Haskell Worked Example
+To help you get over the confidence barrier, try out this skeletal worked example.
+ First cut and paste the two modules below into "Main.hs" and "Printf.hs":
+
+
+{- Main.hs -}
+module Main where
+
+-- Import our template "pr"
+import Printf ( pr )
+
+-- The splice operator $ takes the Haskell source code
+-- generated at compile time by "pr" and splices it into
+-- the argument of "putStrLn".
+main = putStrLn ( $(pr "Hello") )
+
+
+
+{- Printf.hs -}
+module Printf where
+
+-- Skeletal printf from the paper.
+-- It needs to be in a separate module to the one where
+-- you intend to use it.
+
+-- Import some Template Haskell syntax
+import Language.Haskell.THSyntax
+
+-- Describe a format string
+data Format = D | S | L String
+
+-- Parse a format string. This is left largely to you
+-- as we are here interested in building our first ever
+-- Template Haskell program and not in building printf.
+parse :: String -> [Format]
+parse s = [ L s ]
+
+-- Generate Haskell source code from a parsed representation
+-- of the format string. This code will be spliced into
+-- the module which calls "pr", at compile time.
+gen :: [Format] -> Expr
+gen [D] = [| \n -> show n |]
+gen [S] = [| \s -> s |]
+gen [L s] = string s
+
+-- Here we generate the Haskell code for the splice
+-- from an input format string.
+pr :: String -> Expr
+pr s = gen (parse s)
+
+
+Now run the compiler (here we are using a "stage three" build of GHC, at a Cygwin prompt on Windows):
+
+
+ghc/compiler/stage3/ghc-inplace --make -fglasgow-exts -package haskell-src main.hs -o main.exe
+
+
+Run "main.exe" and here is your output:
+
+
+
+$ ./main
+Hello
+
+
+
+
+
+Arrow notation
+
+
+Arrows are a generalization of monads introduced by John Hughes.
+For more details, see
+
+
+
+
+“Generalising Monads to Arrows”,
+John Hughes, in Science of Computer Programming 37,
+pp67–111, May 2000.
+
+
+
+
+
+“A New Notation for Arrows”,
+Ross Paterson, in ICFP, Sep 2001.
+
+
+
+
+
+“Arrows and Computation”,
+Ross Paterson, in The Fun of Programming,
+Palgrave, 2003.
+
+
+
+
+and the arrows web page at
+http://www.haskell.org/arrows/.
+With the flag, GHC supports the arrow
+notation described in the second of these papers.
+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
+module.
+
+
+The extension adds a new kind of expression for defining arrows,
+of the form proc pat -> cmd,
+where proc is a new keyword.
+The variables of the pattern are bound in the body of the
+proc-expression,
+which is a new sort of thing called a command.
+The syntax of commands is as follows:
+
+cmd ::= exp1 -< exp2
+ | exp1 -<< exp2
+ | do { cstmt1 .. cstmtn ; cmd }
+ | let decls in cmd
+ | if exp then cmd1 else cmd2
+ | case exp of { calts }
+ | cmd1 qop cmd2
+ | (| exp |) cmd1 .. cmdn
+ | \ pat1 .. patn -> cmd
+ | ( cmd )
+
+cstmt ::= let decls
+ | pat <- cmd
+ | rec { cstmt1 .. cstmtn }
+ | cmd
+
+Commands produce values, but (like monadic computations)
+may yield more than one value,
+or none, and may do other things as well.
+For the most part, familiarity with monadic notation is a good guide to
+using commands.
+However the values of expressions, even monadic ones,
+are determined by the values of the variables they contain;
+this is not necessarily the case for commands.
+
+
+
+A simple example of the new notation is the expression
+
+proc x -> f -< x+1
+
+We call this a procedure or
+arrow abstraction.
+As with a lambda expression, the variable x
+is a new variable bound within the proc-expression.
+It refers to the input to the arrow.
+In the above example, -< is not an identifier but an
+new reserved symbol used for building commands from an expression of arrow
+type and an expression to be fed as input to that arrow.
+(The weird look will make more sense later.)
+It may be read as analogue of application for arrows.
+The above example is equivalent to the Haskell expression
+
+arr (\ x -> x+1) >>> f
+
+That would make no sense if the expression to the left of
+-< involves the bound variable x.
+More generally, the expression to the left of -<
+may not involve any local variable,
+i.e. a variable bound in the current arrow abstraction.
+For such a situation there is a variant -<<, as in
+
+proc x -> f x -<< x+1
+
+which is equivalent to
+
+arr (\ x -> (f, x+1)) >>> app
+
+so in this case the arrow must belong to the ArrowApply
+class.
+Such an arrow is equivalent to a monad, so if you're using this form
+you may find a monadic formulation more convenient.
+
+
+
+do-notation for commands
+
+
+Another form of command is a form of do-notation.
+For example, you can write
+
+proc x -> do
+ y <- f -< x+1
+ g -< 2*y
+ let z = x+y
+ t <- h -< x*z
+ returnA -< t+z
+
+You can read this much like ordinary do-notation,
+but with commands in place of monadic expressions.
+The first line sends the value of x+1 as an input to
+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
+module as arr id.
+The above example is treated as an abbreviation for
+
+arr (\ x -> (x, x)) >>>
+ first (arr (\ x -> x+1) >>> f) >>>
+ arr (\ (y, x) -> (y, (x, y))) >>>
+ first (arr (\ y -> 2*y) >>> g) >>>
+ arr snd >>>
+ arr (\ (x, y) -> let z = x+y in ((x, z), z)) >>>
+ first (arr (\ (x, z) -> x*z) >>> h) >>>
+ arr (\ (t, z) -> t+z) >>>
+ returnA
+
+Note that variables not used later in the composition are projected out.
+After simplification using rewrite rules (see )
+defined in the
+Control.Arrow
+module, this reduces to
+
+arr (\ x -> (x+1, x)) >>>
+ first f >>>
+ arr (\ (y, x) -> (2*y, (x, y))) >>>
+ first g >>>
+ arr (\ (_, (x, y)) -> let z = x+y in (x*z, z)) >>>
+ first h >>>
+ arr (\ (t, z) -> t+z)
+
+which is what you might have written by hand.
+With arrow notation, GHC keeps track of all those tuples of variables for you.
+
+
+
+Note that although the above translation suggests that
+let-bound variables like z must be
+monomorphic, the actual translation produces Core,
+so polymorphic variables are allowed.
+
+
+
+It's also possible to have mutually recursive bindings,
+using the new rec keyword, as in the following example:
+
+counter :: ArrowCircuit a => a Bool Int
+counter = proc reset -> do
+ rec output <- returnA -< if reset then 0 else next
+ next <- delay 0 -< output+1
+ returnA -< output
+
+The translation of such forms uses the loop combinator,
+so the arrow concerned must belong to the ArrowLoop class.
+
+
+
+
+
+Conditional commands
+
+
+In the previous example, we used a conditional expression to construct the
+input for an arrow.
+Sometimes we want to conditionally execute different commands, as in
+
+proc (x,y) ->
+ if f x y
+ then g -< x+1
+ else h -< y+2
+
+which is translated to
+
+arr (\ (x,y) -> if f x y then Left x else Right y) >>>
+ (arr (\x -> x+1) >>> f) ||| (arr (\y -> y+2) >>> g)
+
+Since the translation uses |||,
+the arrow concerned must belong to the ArrowChoice class.
+
+
+
+There are also case commands, like
+
+case input of
+ [] -> f -< ()
+ [x] -> g -< x+1
+ x1:x2:xs -> do
+ y <- h -< (x1, x2)
+ ys <- k -< xs
+ returnA -< y:ys
+
+The syntax is the same as for case expressions,
+except that the bodies of the alternatives are commands rather than expressions.
+The translation is similar to that of if commands.
+
+
+
+
+
+Defining your own control structures
+
+
+As we're seen, arrow notation provides constructs,
+modelled on those for expressions,
+for sequencing, value recursion and conditionals.
+But suitable combinators,
+which you can define in ordinary Haskell,
+may also be used to build new commands out of existing ones.
+The basic idea is that a command defines an arrow from environments to values.
+These environments assign values to the free local variables of the command.
+Thus combinators that produce arrows from arrows
+may also be used to build commands from commands.
+For example, the ArrowChoice class includes a combinator
+
+ArrowChoice a => (<+>) :: a e c -> a e c -> a e c
+
+so we can use it to build commands:
+
+expr' = proc x ->
+ returnA -< x
+ <+> do
+ symbol Plus -< ()
+ y <- term -< ()
+ expr' -< x + y
+ <+> do
+ symbol Minus -< ()
+ y <- term -< ()
+ expr' -< x - y
+
+This is equivalent to
+
+expr' = (proc x -> returnA -< x)
+ <+> (proc x -> do
+ symbol Plus -< ()
+ y <- term -< ()
+ expr' -< x + y)
+ <+> (proc x -> do
+ symbol Minus -< ()
+ y <- term -< ()
+ expr' -< x - y)
+
+It is essential that this operator be polymorphic in e
+(representing the environment input to the command
+and thence to its subcommands)
+and satisfy the corresponding naturality property
+
+arr k >>> (f <+> g) = (arr k >>> f) <+> (arr k >>> g)
+
+at least for strict k.
+(This should be automatic if you're not using seq.)
+This ensures that environments seen by the subcommands are environments
+of the whole command,
+and also allows the translation to safely trim these environments.
+The operator must also not use any variable defined within the current
+arrow abstraction.
+
+
+
+We could define our own operator
+
+untilA :: ArrowChoice a => a e () -> a e Bool -> a e ()
+untilA body cond = proc x ->
+ if cond x then returnA -< ()
+ else do
+ body -< x
+ untilA body cond -< x
+
+and use it in the same way.
+Of course this infix syntax only makes sense for binary operators;
+there is also a more general syntax involving special brackets:
+
+proc x -> do
+ y <- f -< x+1
+ (|untilA|) (increment -< x+y) (within 0.5 -< x)
+
+
+
+
+Some operators will need to pass additional inputs to their subcommands.
+For example, in an arrow type supporting exceptions,
+the operator that attaches an exception handler will wish to pass the
+exception that occurred to the handler.
+Such an operator might have a type
+
+handleA :: ... => a e c -> a (e,Ex) c -> a e c
+
+where Ex is the type of exceptions handled.
+You could then use this with arrow notation by writing a command
+
+body `handleA` \ ex -> handler
+
+so that if an exception is raised in the command body,
+the variable ex is bound to the value of the exception
+and the command handler,
+which typically refers to ex, is entered.
+Though the syntax here looks like a functional lambda,
+we are talking about commands, and something different is going on.
+The input to the arrow represented by a command consists of values for
+the free local variables in the command, plus a stack of anonymous values.
+In all the prior examples, this stack was empty.
+In the second argument to handleA,
+this stack consists of one value, the value of the exception.
+The command form of lambda merely gives this value a name.
+
+
+
+More concretely,
+the values on the stack are paired to the right of the environment.
+So when designing operators like handleA that pass
+extra inputs to their subcommands,
+More precisely, the type of each argument of the operator (and its result)
+should have the form
+
+a (...(e,t1), ... tn) t
+
+where e is the polymorphic variable
+(representing the environment)
+and ti are the types of the values on the stack,
+with t1 being the top.
+The polymorphic variable e must not occur in
+a, ti or
+t.
+However the arrows involved need not be the same.
+Here are some more examples of suitable operators:
+
+bracketA :: ... => a e b -> a (e,b) c -> a (e,c) d -> a e d
+runReader :: ... => a e c -> a' (e,State) c
+runState :: ... => a e c -> a' (e,State) (c,State)
+
+How can we supply the extra input required by the last two?
+We can define yet another operator, a counterpart of the monadic
+>>= operator:
+
+bind :: Arrow a => a e b -> a (e,b) c -> a e c
+u `bind` f = returnA &&& u >>> f
+
+and then build commands like
+
+proc x ->
+ (mkState -< x) `bind` (|runReader|) (do { ... })
+
+which uses the arrow mkState to create a state,
+and then provides this as an extra input to the command built using
+runReader.
+
+
+
+
+
+Differences with the paper
+
+
+
+
+Instead of a single form of arrow application (arrow tail) with two
+translations, the implementation provides two forms
+-< (first-order)
+and -<< (higher-order).
+
+
+
+
+User-defined operators are flagged with banana brackets instead of
+a new form keyword.
+
+
+
+
+
+
+
+
+Portability
+
+
+Although only GHC implements arrow notation directly,
+there is also a preprocessor
+(available from the
+arrows web page>)
+that translates arrow notation into Haskell 98
+for use with other Haskell systems.
+You would still want to check arrow programs with GHC;
+tracing type errors in the preprocessor output is not easy.
+Modules intended for both GHC and the preprocessor must observe some
+additional restrictions:
+
+
+
+
+The module must import
+Control.Arrow.
+
+
+
+
+
+The preprocessor cannot cope with other Haskell extensions.
+These would have to go in separate modules.
+
+
+
+
+
+Because the preprocessor targets Haskell (rather than Core),
+let-bound variables are monomorphic.
+
+
+
+
+
+
+
+
+
+
@@ -3399,32 +3908,64 @@ Assertion failures can be caught, see the documentation for the
unrecognised word is (silently)
ignored.
-
-INLINE pragma
+ <sect2 id="deprecated-pragma">
+ <title>DEPRECATED pragma
+ DEPRECATED
+
-INLINE pragma
-pragma, INLINE
+ The DEPRECATED pragma lets you specify that a particular
+ function, class, or type, is deprecated. There are two
+ forms.
-
-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 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 will probably see these unfoldings (in Core syntax) in your
-interface files.
-
+
+ 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.
+
+
-
-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.
-
+ You can suppress the warnings with the flag
+ .
+
-
-The sledgehammer you can bring to bear is the
-INLINEINLINE pragma pragma, used thusly:
+
+ INLINE and NOINLINE pragmas
+
+ These pragmas control the inlining of function
+ definitions.
+
+
+ INLINE pragma
+ INLINE
+
+ 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.
+ 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:
key_function :: Int -> String -> (Bool, Double)
@@ -3434,25 +3975,25 @@ key_function :: Int -> String -> (Bool, Double)
#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.)
-
+ (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.)
-
-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.
-
+ 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
-signature could be put.
-
+ Syntactially, 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__
@@ -3461,32 +4002,140 @@ For example, in GHC's own UniqueSupply monad code, we have:
#endif
-
+ See also the NOINLINE pragma ().
+
+
+
+ NOINLINE pragma
+
+ NOINLINE
+ NOTINLINE
+
+ 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.
+
+ 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).
+
+
+
+ Phase control
+
+ Sometimes you want to control exactly when in GHC's
+ pipeline the INLINE pragma is switched on. Inlining happens
+ only during runs of the simplifier. Each
+ run of the simplifier has a different phase
+ number; the phase number decreases towards zero.
+ If you use you'll see the
+ sequence of phase numbers for successive runs of the
+ simpifier. In an INLINE pragma you can optionally specify a
+ phase number, thus:
-
+
+
+ You can say "inline f in Phase 2
+ and all subsequent phases":
+
+ {-# INLINE [2] f #-}
+
+
+
-
-NOINLINE pragma
-
+
+ You can say "inline g in all
+ phases up to, but not including, Phase 3":
+
+ {-# INLINE [~3] g #-}
+
+
+
-NOINLINE pragma
-pragmaNOINLINE
-NOTINLINE pragma
-pragmaNOTINLINE
+
+ If you omit the phase indicator, you mean "inline in
+ all phases".
+
+
-
-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.
-
+ You can use a phase number on a NOINLINE pragma too:
-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).
+
+
+ You can say "do not inline f
+ until Phase 2; in Phase 2 and subsequently behave as if
+ there was no pragma at all":
+
+ {-# NOINLINE [2] f #-}
+
+
+
-
+
+ You can say "do not inline g in
+ Phase 3 or any subsequent phase; before that, behave as if
+ there was no pragma":
+
+ {-# NOINLINE [~3] g #-}
+
+
+
+
+
+ If you omit the phase indicator, you mean "never
+ inline this function".
+
+
+
+ The same phase-numbering control is available for RULES
+ ().
+
+
+
+
+ LINE pragma
+
+ LINEpragma
+ pragmaLINE
+ 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 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
+ pragma.
+
+
+
+ OPTIONS pragma
+ OPTIONS
+
+ pragmaOPTIONS
+
+
+ The OPTIONS pragma is used to specify
+ additional options that are given to the compiler when compiling
+ this source file. See for
+ details.
+
+
+
+ RULES pragma
+
+ The RULES pragma lets you specify rewrite rules. It is
+ described in .
+ SPECIALIZE pragma
@@ -3512,11 +4161,14 @@ hammeredLookup :: Ord key => [(key, value)] -> key -> value
{-# SPECIALIZE hammeredLookup :: [(Widget, value)] -> Widget -> value #-}
+ A SPECIALIZE pragma for a function can
+ be put anywhere its type signature could be put.
+
To get very fancy, you can also specify a named function
to use for the specialised value, as in:
-{-# RULES hammeredLookup = blah #-}
+{-# RULES "hammeredLookup" hammeredLookup = blah #-}
where blah is an implementation of
@@ -3539,7 +4191,7 @@ hammeredLookup :: Ord key => [(key, value)] -> key -> value
toDouble :: Real a => a -> Double
toDouble = fromRational . toRational
-{-# SPECIALIZE toDouble :: Int -> Double = i2d #-}
+{-# RULES "toDouble/Int" toDouble = i2d #-}
i2d (I# i) = D# (int2Double# i) -- uses Glasgow prim-op directly
@@ -3548,9 +4200,6 @@ i2d (I# i) = D# (int2Double# i) -- uses Glasgow prim-op directly
Rational—is obscenely expensive by
comparison.
- A SPECIALIZE pragma for a function can
- be put anywhere its type signature could be put.
-
@@ -3578,83 +4227,7 @@ of the pragma.
-
-LINE pragma
-
-
-LINE pragma
-pragma, LINE
-
-
-
-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
-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
-pragma.
-
-
-
-
-
-RULES pragma
-
-
-The RULES pragma lets you specify rewrite rules. It is described in
-.
-
-
-
-
-
-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 .
-
-
@@ -3693,16 +4266,34 @@ From a syntactic point of view:
+ There may be zero or more rules in a RULES pragma.
+
+
+
+
+
+
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.
+A rule may optionally have a phase-control number (see ),
+immediately after the name of the rule. Thus:
+
+ {-# RULES
+ "map/map" [2] forall f g xs. map f (map g xs) = map (f.g) xs
+ #-}
+
+The "[2]" means that the rule is active in Phase 2 and subsequent phases. The inverse
+notation "[~2]" is also accepted, meaning that the rule is active up to, but not including,
+Phase 2.
+
+
@@ -3711,6 +4302,7 @@ is set, so you must lay out your rules starting in the same column as the
enclosing definitions.
+
@@ -4186,7 +4778,7 @@ 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 GHC/Base.lhs looks llike this:
build :: forall a. (forall b. (a -> b -> b) -> b -> b) -> [a]
@@ -4204,9 +4796,9 @@ in the RHS of the INLINE thing. I regret the delicacy of thi
- In ghc/lib/std/PrelBase.lhs look at the rules for map to
+ In libraries/base/GHC/Base.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 GHC/List.lhs.
@@ -4216,6 +4808,69 @@ program even if fusion doesn't happen. More rules in PrelList.lhs
+
+ CORE pragma
+
+ CORE pragma
+ pragma, CORE
+ core, annotation
+
+
+ The external core format supports Note annotations;
+ the CORE pragma gives a way to specify what these
+ should be in your Haskell source code. Syntactically, core
+ annotations are attached to expressions and take a Haskell string
+ literal as an argument. The following function definition shows an
+ example:
+
+
+f x = ({-# CORE "foo" #-} show) ({-# CORE "bar" #-} x)
+
+
+ Sematically, this is equivalent to:
+
+
+g x = show x
+
+
+
+
+ However, when external for is generated (via
+ ), there will be Notes attached to the
+ expressions show and x.
+ The core function declaration for f is:
+
+
+
+ f :: %forall a . GHCziShow.ZCTShow a ->
+ a -> GHCziBase.ZMZN GHCziBase.Char =
+ \ @ a (zddShow::GHCziShow.ZCTShow a) (eta::a) ->
+ (%note "foo"
+ %case zddShow %of (tpl::GHCziShow.ZCTShow a)
+ {GHCziShow.ZCDShow
+ (tpl1::GHCziBase.Int ->
+ a ->
+ GHCziBase.ZMZN GHCziBase.Char -> GHCziBase.ZMZN GHCziBase.Cha
+r)
+ (tpl2::a -> GHCziBase.ZMZN GHCziBase.Char)
+ (tpl3::GHCziBase.ZMZN a ->
+ GHCziBase.ZMZN GHCziBase.Char -> GHCziBase.ZMZN GHCziBase.Cha
+r) ->
+ tpl2})
+ (%note "foo"
+ eta);
+
+
+
+ Here, we can see that the function show (which
+ has been expanded out to a case expression over the Show dictionary)
+ has a %note attached to it, as does the
+ expression eta (which used to be called
+ x).
+
+
+
+
@@ -4264,7 +4919,7 @@ 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
+That is, just leave off the "where" clause. Of course, you can put in the
where clause and over-ride whichever methods you please.