X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=docs%2Fusers_guide%2Fglasgow_exts.xml;h=f8ad5c1b0ac8a79f9ee96c6a4227e6abafcaa6e7;hp=110679070b048a548e4a52e7aa78e9e622c349fc;hb=9da4639011348fb6c318e3cba4b08622f811d9c4;hpb=5d7b55731e31c04ba76d670d0176e32f121fc5e4
diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml
index 1106790..f8ad5c1 100644
--- a/docs/users_guide/glasgow_exts.xml
+++ b/docs/users_guide/glasgow_exts.xml
@@ -905,6 +905,38 @@ fromInteger :: Integer -> Bool -> Bool
you should be all right.
+
+
+Postfix operators
+
+
+GHC allows a small extension to the syntax of left operator sections, which
+allows you to define postfix operators. The extension is this: the left section
+
+ (e !)
+
+is equivalent (from the point of view of both type checking and execution) to the expression
+
+ ((!) e)
+
+(for any expression e and operator (!).
+The strict Haskell 98 interpretation is that the section is equivalent to
+
+ (\y -> (!) e y)
+
+That is, the operator must be a function of two arguments. GHC allows it to
+take only one argument, and that in turn allows you to write the function
+postfix.
+
+Since this extension goes beyond Haskell 98, it should really be enabled
+by a flag; but in fact it is enabled all the time. (No Haskell 98 programs
+change their behaviour, of course.)
+
+The extension does not extend to the left-hand side of function
+definitions; you must define such a function in prefix form.
+
+
+
@@ -3236,7 +3268,7 @@ f xs = ys ++ ys
The type signature for f brings the type variable a into scope; it scopes over
the entire definition of f.
-In particular, it is in scope at the type signature for y.
+In particular, it is in scope at the type signature for ys.
In Haskell 98 it is not possible to declare
a type for ys; a major benefit of scoped type variables is that
it becomes possible to do so.
@@ -3269,6 +3301,7 @@ changing the program.
A lexically scoped type variable can be bound by:
A declaration type signature ()
+An expression type signature ()
A pattern type signature ()
Class and instance declarations ()
@@ -3320,6 +3353,23 @@ quantification rules.
+
+Expression type signatures
+
+An expression type signature that has explicit
+quantification (using forall) brings into scope the
+explicitly-quantified
+type variables, in the annotated expression. For example:
+
+ f = runST ( (op >>= \(x :: STRef s Int) -> g x) :: forall s. ST s Bool )
+
+Here, the type signature forall a. ST s Bool brings the
+type variable s into scope, in the annotated expression
+(op >>= \(x :: STRef s Int) -> g x).
+
+
+
+
Pattern type signatures
@@ -3328,7 +3378,7 @@ signature.
For example:
-- f and g assume that 'a' is already in scope
- f = \(x::Int, y) -> x
+ f = \(x::Int, y::a) -> x
g (x::a) = x
h ((x,y) :: (Int,Bool)) = (y,x)
@@ -3608,16 +3658,19 @@ declaration (after expansion of any type synonyms)
where
- The type t is an arbitrary type
+ 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.
- The vk+1...vn are type variables which do not occur in
- t, and
+ The k is chosen so that ci (T v1...vk) is well-kinded.
- 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.
+ The type t is an arbitrary type.
+
+
+ The type variables vk+1...vn do not occur in t,
+ nor in the ci, and
None of the ci is Read, Show,
@@ -3630,13 +3683,8 @@ where
Then, for each ci, the derived instance
declaration is:
- instance ci (t vk+1...v) => ci (T v1...vp)
+ instance ci t => ci (T v1...vk)
-where p is chosen so that T v1...vp is of the
-right kind for the last parameter of class Ci.
-
-
-
As an example which does not work, consider
newtype NonMonad m s = NonMonad (State s m s) deriving Monad
@@ -3748,9 +3796,9 @@ pattern binding must have the same context. For example, this is fine:
-Generalised Algebraic Data Types
+Generalised Algebraic Data Types (GADTs)
-Generalised Algebraic Data Types (GADTs) generalise ordinary algebraic data types by allowing you
+Generalised Algebraic Data Types generalise ordinary algebraic data types by allowing you
to give the type signatures of constructors explicitly. For example:
data Term a where
@@ -3771,7 +3819,12 @@ for these Terms:
eval (If b e1 e2) = if eval b then eval e1 else 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.
+These and many other examples are given in papers by Hongwei Xi, and
+Tim Sheard. There is a longer introduction
+on the wiki,
+and Ralf Hinze's
+Fun with phantom types also has a number of examples. Note that papers
+may use different notation to that implemented in GHC.
The rest of this section outlines the extensions to GHC that support GADTs.
@@ -3871,8 +3924,8 @@ 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
+ Nothing1 :: Maybe1 a ;
+ Just1 :: a -> Maybe1 a
} deriving( Eq, Ord )
data Maybe2 a = Nothing2 | Just2 a
@@ -6030,7 +6083,7 @@ r)
GHCziBase.ZMZN GHCziBase.Char -> GHCziBase.ZMZN GHCziBase.Cha
r) ->
tpl2})
- (%note "foo"
+ (%note "bar"
eta);
@@ -6132,9 +6185,6 @@ that it is well typed.
Generic classes
- (Note: support for generic classes is currently broken in
- GHC 5.02).
-
The ideas behind this extension are described in detail in "Derivable type classes",
Ralf Hinze and Simon Peyton Jones, Haskell Workshop, Montreal Sept 2000, pp94-105.