X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=docs%2Fusers_guide%2Fglasgow_exts.xml;h=a1cf5c5f6ab203aff6bfa5174c4b0ce3ee82956a;hb=836cafeb3edd9c728fe4c86cf90229e1476ad14a;hp=80c40087a066679209616befad389e5cd70ee61e;hpb=0093a2827f6b4007c4fcb298a559c9b7dd17aec1;p=ghc-hetmet.git diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index 80c4008..a1cf5c5 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -319,7 +319,7 @@ it is always correct! It is also intended for processing into text. Indeed, the result of such processing is part of the description of the External + url="http://www.haskell.org/ghc/docs/papers/core.ps.gz">External Core language. So that document is a good place to look for a type-set version. We would be very happy if someone wanted to volunteer to produce an SGML @@ -993,7 +993,7 @@ and improve termination (Section 3.2 of the paper). -The web page: http://www.cse.ogi.edu/PacSoft/projects/rmb +The web page: http://www.cse.ogi.edu/PacSoft/projects/rmb/ contains up to date information on recursive monadic bindings. @@ -1689,8 +1689,8 @@ adding a new existential quantification construct. - -Type classes + +Existentials and type classes An easy extension is to allow @@ -2016,19 +2016,8 @@ In the example, the equality dictionary is used to satisfy the equality constrai generated by the call to elem, so that the type of insert itself has no Eq constraint. -This behaviour contrasts with Haskell 98's peculiar treatment of -contexts on a data type declaration (Section 4.2.1 of the Haskell 98 Report). -In Haskell 98 the definition - - data Eq a => Set' a = MkSet' [a] - -gives MkSet' the same type as MkSet above. But instead of -making available an (Eq a) constraint, pattern-matching -on MkSet' requires an (Eq a) constraint! -GHC faithfully implements this behaviour, odd though it is. But for GADT-style declarations, -GHC's behaviour is much more useful, as well as much more intuitive. -For example, a possible application of GHC's behaviour is to reify dictionaries: +For example, one possible application is to reify dictionaries: data NumInst a where MkNumInst :: Num a => NumInst a @@ -2042,6 +2031,38 @@ For example, a possible application of GHC's behaviour is to reify dictionaries: Here, a value of type NumInst a is equivalent to an explicit (Num a) dictionary. + +All this applies to constructors declared using the syntax of . +For example, the NumInst data type above could equivalently be declared +like this: + + data NumInst a + = Num a => MkNumInst (NumInst a) + +Notice that, unlike the situation when declaring an existental, there is +no forall, because the Num constrains the +data type's univerally quantified type variable a. +A constructor may have both universal and existential type variables: for example, +the following two declarations are equivalent: + + data T1 a + = forall b. (Num a, Eq b) => MkT1 a b + data T2 a where + MkT2 :: (Num a, Eq b) => a -> b -> T2 a + + +All this behaviour contrasts with Haskell 98's peculiar treatment of +contexts on a data type declaration (Section 4.2.1 of the Haskell 98 Report). +In Haskell 98 the definition + + data Eq a => Set' a = MkSet' [a] + +gives MkSet' the same type as MkSet above. But instead of +making available an (Eq a) constraint, pattern-matching +on MkSet' requires an (Eq a) constraint! +GHC faithfully implements this behaviour, odd though it is. But for GADT-style declarations, +GHC's behaviour is much more useful, as well as much more intuitive. + The rest of this section gives further details about GADT-style data @@ -2193,7 +2214,7 @@ the type a is refined to Int. That's the A precise specification of the type rules is beyond what this user manual aspires to, but the design closely follows that described in the paper Simple +url="http://research.microsoft.com/%7Esimonpj/papers/gadt/">Simple unification-based type inference for GADTs, (ICFP 2006). The general principle is this: type refinement is only carried out @@ -2212,7 +2233,7 @@ the result type of the case expression. Hence the addition < These and many other examples are given in papers by Hongwei Xi, and Tim Sheard. There is a longer introduction -on the wiki, +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. @@ -2588,8 +2609,8 @@ the standard method is used or the one described here.) 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 +url="http://research.microsoft.com/~simonpj/Papers/type-class-design-space/">Type +classes: exploring the design space (Simon Peyton Jones, Mark Jones, Erik Meijer). @@ -2668,7 +2689,7 @@ 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. +GHC lifts this restriction (flag ). @@ -2680,7 +2701,7 @@ GHC lifts this restriction. Functional dependencies are implemented as described by Mark Jones -in “Type Classes with Functional Dependencies”, Mark P. 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, . @@ -3015,7 +3036,7 @@ must be of the form C a where a is a type variable that occurs in the head. -The flag loosens these restrictions +The flag loosens these restrictions considerably. Firstly, multi-parameter type classes are permitted. Secondly, the context and head of the instance declaration can each consist of arbitrary (well-kinded) assertions (C t1 ... tn) subject only to the @@ -3380,7 +3401,7 @@ instance of Num or of IsString< -The standard defaulting rule (Haskell Report, Section 4.3.4) +The standard defaulting rule (Haskell Report, Section 4.3.4) is extended thus: defaulting applies when all the unresolved constraints involve standard classes or IsString; and at least one is a numeric class or IsString. @@ -3421,7 +3442,7 @@ to work since it gets translated into an equality comparison. Type signatures -The context of a type signature +The context of a type signature Unlike Haskell 98, constraints in types do not have to be of the form (class type-variable) or @@ -4057,9 +4078,18 @@ The function f3 has a rank-3 type; it has rank-2 types on the left of a function arrow. -GHC allows types of arbitrary rank; you can nest foralls -arbitrarily deep in function arrows. (GHC used to be restricted to rank 2, but -that restriction has now been lifted.) +GHC has three flags to control higher-rank types: + + + : data constructors (only) can have polymorphic argment types. + + + : any function (including data constructors) can have a rank-2 type. + + + : any function (including data constructors) can have an arbitrary-rank type. +That is, you can nest foralls +arbitrarily deep in function arrows. In particular, a forall-type (also called a "type scheme"), including an operational type class context, is legal: @@ -4071,6 +4101,8 @@ field type signatures. As the type of an implicit parameter In a pattern type signature (see ) + + Of course forall becomes a keyword; you can't use forall as a type variable any more! @@ -4322,7 +4354,7 @@ Notice here that the Maybe type is parameterised by the [a]). The technical details of this extension are described in the paper -Boxy types: +Boxy types: type inference for higher-rank types and impredicativity, which appeared at ICFP 2006. @@ -4385,7 +4417,7 @@ A lexically scoped type variable can be bound by: In Haskell, a programmer-written type signature is implicitly quantified over its free type variables (Section +url="http://www.haskell.org/onlinereport/decls.html#sect4.1.2">Section 4.1.2 of the Haskel Report). Lexically scoped type variables affect this implicit quantification rules @@ -4450,7 +4482,7 @@ type variable s into scope, in the annotated expression Pattern type signatures A type signature may occur in any pattern; this is a pattern type -signature. +signature. For example: -- f and g assume that 'a' is already in scope @@ -4463,9 +4495,27 @@ already in scope (i.e. bound by the enclosing context), matters are simple: the signature simply constrains the type of the pattern in the obvious way. -There is only one situation in which you can write a pattern type signature that -mentions a type variable that is not already in scope, namely in pattern match -of an existential data constructor. For example: +Unlike expression and declaration type signatures, pattern type signatures are not implictly generalised. +The pattern in a patterm binding may only mention type variables +that are already in scope. For example: + + f :: forall a. [a] -> (Int, [a]) + f xs = (n, zs) + where + (ys::[a], n) = (reverse xs, length xs) -- OK + zs::[a] = xs ++ ys -- OK + + Just (v::b) = ... -- Not OK; b is not in scope + +Here, the pattern signatures for ys and zs +are fine, but the one for v is not because b is +not in scope. + + +However, in all patterns other than pattern bindings, a pattern +type signature may mention a type variable that is not in scope; in this case, +the signature brings that type variable into scope. +This is particularly important for existential data constructors. For example: data T = forall a. MkT [a] @@ -4475,14 +4525,19 @@ of an existential data constructor. For example: t3::[a] = [t,t,t] Here, the pattern type signature (t::a) mentions a lexical type -variable that is not already in scope. Indeed, it cannot already be in scope, +variable that is not already in scope. Indeed, it cannot already be in scope, because it is bound by the pattern match. GHC's rule is that in this situation (and only then), a pattern type signature can mention a type variable that is not already in scope; the effect is to bring it into scope, standing for the existentially-bound type variable. -If this seems a little odd, we think so too. But we must have +When a pattern type signature binds a type variable in this way, GHC insists that the +type variable is bound to a rigid, or fully-known, type variable. +This means that any user-written type signature always stands for a completely known type. + + +If all this seems a little odd, we think so too. But we must have some way to bring such type variables into scope, else we could not name existentially-bound type variables in subsequent type signatures. @@ -4580,18 +4635,18 @@ scope over the methods defined in the where part. For exampl 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 +(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). +(Haskell Report, Section 4.5.2). Following a suggestion of Mark Jones, in his paper -Typing Haskell in +Typing Haskell in Haskell, GHC implements a more general scheme. If is specified: @@ -4652,7 +4707,7 @@ Currently, only the former are fully implemented, while we are still working on the latter. As a result, the specification of the language extension is also still to some degree in flux. Hence, a more detailed description of the language extension and its use is currently available -from the Haskell +from the Haskell wiki page on type families. The material will be moved to this user's guide when it has stabilised. @@ -4675,12 +4730,12 @@ Type families are enabled by the flag . Haskell. The background to the main technical innovations is discussed in " +url="http://research.microsoft.com/~simonpj/papers/meta-haskell/"> Template Meta-programming for Haskell" (Proc Haskell Workshop 2002). There is a Wiki page about -Template Haskell at +Template Haskell at http://www.haskell.org/haskellwiki/Template_Haskell, and that is the best place to look for further details. You may also @@ -5503,7 +5558,7 @@ prefix notation: (!) f x = 3 The semantics of Haskell pattern matching is described in +url="http://www.haskell.org/onlinereport/exps.html#sect3.17.2"> Section 3.17.2 of the Haskell Report. To this description add one extra item 10, saying: Matching @@ -5513,7 +5568,7 @@ the pattern !pat against a value v behaves v -Similarly, in Figure 4 of +Similarly, in Figure 4 of Section 3.17.3, add a new case (t): case v of { !pat -> e; _ -> e' } @@ -5521,7 +5576,7 @@ case v of { !pat -> e; _ -> e' } That leaves let expressions, whose translation is given in -Section +Section 3.12 of the Haskell Report. In the translation box, first apply @@ -5835,8 +5890,26 @@ key_function :: Int -> String -> (Bool, Double) 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. - + inline it. However, an INLINE pragma for a + function "f" has a number of other effects: + + +No funtions are inlined into f. Otherwise +GHC might inline a big function into f's right hand side, +making f big; and then inline f blindly. + + +The float-in, float-out, and common-sub-expression transformations are not +applied to the body of f. + + +An INLINE function is not worker/wrappered by strictness analysis. +It's going to be inlined wholesale instead. + + +All of these effects are aimed at ensuring that what gets inlined is +exactly what you asked for, no more and no less. + Syntactically, an INLINE pragma for a function can be put anywhere its type signature could be put. @@ -7098,7 +7171,7 @@ carried out at let and where bindings. Haskell's monomorphism restriction (see -Section +Section 4.5.5 of the Haskell Report) can be completely switched off by @@ -7125,7 +7198,7 @@ can be completely switched off by [x] = e -- A pattern binding Experimentally, GHC now makes pattern bindings monomorphic by -default. Use to recover the +default. Use to recover the standard behaviour.