X-Git-Url: http://git.megacz.com/?p=ghc-hetmet.git;a=blobdiff_plain;f=docs%2Fusers_guide%2Fglasgow_exts.xml;h=372ebab6fcfee28552cd754a08da78cd612c5928;hp=24b049ec36fce3d66d4f1205eda9a4af222751ea;hb=25f84fa7e4b84c3db5ba745a7881c009b778e0b1;hpb=f416411e20d042bbec7946cd71ccefa7483e42a7 diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index 24b049e..372ebab 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -38,11 +38,21 @@ documentation describes all the libraries that come with GHC. extensionsoptions controlling - These flags control what variation of the language are + The language option flag control what variation of the language are permitted. Leaving out all of them gives you standard Haskell 98. - NB. turning on an option that enables special syntax + Generally speaking, all the language options are introduced by "", + e.g. . + + + All the language options can be turned off by using the prefix ""; + e.g. "". + + Language options recognised by Cabal can also be enabled using the LANGUAGE pragma, + thus {-# LANGUAGE TemplateHaskell #-} (see >). + + Turning on an option that enables special syntax might cause working Haskell 98 code to fail to compile, perhaps because it uses a variable name which has become a reserved word. So, together with each option below, we @@ -81,7 +91,8 @@ documentation describes all the libraries that come with GHC. This simultaneously enables all of the extensions to Haskell 98 described in , except where otherwise - noted. + noted. We are trying to move away from this portmanteau flag, + and towards enabling features individaully. New reserved words: forall (only in types), mdo. @@ -95,20 +106,24 @@ documentation describes all the libraries that come with GHC. float##, (#, #), |), {|. + + Implies these specific language options: + , + , + , + , + . - and : - - + : + This option enables the language extension defined in the - Haskell 98 Foreign Function Interface Addendum plus deprecated - syntax of previous versions of the FFI for backwards - compatibility. + Haskell 98 Foreign Function Interface Addendum. New reserved words: foreign. @@ -116,7 +131,7 @@ documentation describes all the libraries that come with GHC. - ,: + ,: These two flags control how generalisation is done. @@ -127,8 +142,8 @@ documentation describes all the libraries that come with GHC. - : - + : + Use GHCi's extended default rules in a regular module (). @@ -139,16 +154,16 @@ documentation describes all the libraries that come with GHC. - - + + - - + + - - + + @@ -173,8 +188,8 @@ documentation describes all the libraries that come with GHC. - - + + See . Independent of @@ -192,8 +207,8 @@ documentation describes all the libraries that come with GHC. - - + + See . Independent of @@ -202,13 +217,13 @@ documentation describes all the libraries that come with GHC. - + - -fno-implicit-prelude + -XNoImplicitPrelude option GHC normally imports Prelude.hi files for you. If you'd rather it didn't, then give it a - option. The idea is + option. The idea is that you can then import a Prelude of your own. (But don't call it Prelude; the Haskell module namespace is flat, and you must not conflict with any @@ -223,14 +238,14 @@ documentation describes all the libraries that come with GHC. translation for list comprehensions continues to use Prelude.map etc. - However, does + However, does change the handling of certain built-in syntax: see . - + Enables implicit parameters (see ). Currently also implied by @@ -243,7 +258,15 @@ documentation describes all the libraries that come with GHC. - + + + Enables overloaded string literals (see ). + + + + + Enables lexically-scoped type variables (see ). Implied by @@ -252,7 +275,7 @@ documentation describes all the libraries that come with GHC. - + Enables Template Haskell (see ). This flag must @@ -271,8 +294,6 @@ documentation describes all the libraries that come with GHC. - - Unboxed types and primitive operations @@ -377,6 +398,13 @@ worse, the unboxed value might be larger than a pointer (Double# for instance). + You cannot define a newtype whose representation type +(the argument type of the data constructor) is an unboxed type. Thus, +this is illegal: + + newtype A = MkA Int# + + You cannot bind a variable with an unboxed type in a top-level binding. @@ -546,14 +574,11 @@ import qualified Control.Monad.ST.Strict as ST linkend="search-path"/>. GHC comes with a large collection of libraries arranged - hierarchically; see the accompanying library documentation. - There is an ongoing project to create and maintain a stable set - of core libraries used by several Haskell - compilers, and the libraries that GHC comes with represent the - current status of that project. For more details, see Haskell - Libraries. - + hierarchically; see the accompanying library + documentation. More libraries to install are available + from HackageDB. @@ -622,7 +647,7 @@ to write clunky would be to use case expressions: -clunky env var1 var1 = case lookup env var1 of +clunky env var1 var2 = case lookup env var1 of Nothing -> fail Just val1 -> case lookup env var2 of Nothing -> fail @@ -647,7 +672,7 @@ Here is how I would write clunky: -clunky env var1 var1 +clunky env var1 var2 | Just val1 <- lookup env var1 , Just val2 <- lookup env var2 = val1 + val2 @@ -692,9 +717,11 @@ qualifier list has just one element, a boolean expression. The recursive do-notation (also known as mdo-notation) is implemented as described in -"A recursive do for Haskell", -Levent Erkok, John Launchbury", +A recursive do for Haskell, +by Levent Erkok, John Launchbury, Haskell Workshop 2002, pages: 29-37. Pittsburgh, Pennsylvania. +This paper is essential reading for anyone making non-trivial use of mdo-notation, +and we do not repeat it here. The do-notation of Haskell does not allow recursive bindings, @@ -725,17 +752,24 @@ class Monad m => MonadFix m where The function mfix -dictates how the required recursion operation should be performed. If recursive bindings are required for a monad, -then that monad must be declared an instance of the MonadFix class. -For details, see the above mentioned reference. +dictates how the required recursion operation should be performed. For example, +justOnes desugars as follows: + +justOnes = mfix (\xs' -> do { xs <- Just (1:xs'); return xs } + +For full details of the way in which mdo is typechecked and desugared, see +the paper A recursive do for Haskell. +In particular, GHC implements the segmentation technique described in Section 3.2 of the paper. +If recursive bindings are required for a monad, +then that monad must be declared an instance of the MonadFix class. The following instances of MonadFix are automatically provided: List, Maybe, IO. Furthermore, the Control.Monad.ST and Control.Monad.ST.Lazy modules provide the instances of the MonadFix class for Haskell's internal state monad (strict and lazy, respectively). -There are three important points in using the recursive-do notation: +Here are some important points in using the recursive-do notation: The recursive version of the do-notation uses the keyword mdo (rather @@ -743,14 +777,21 @@ than do). -You should import Control.Monad.Fix. -(Note: Strictly speaking, this import is required only when you need to refer to the name -MonadFix in your program, but the import is always safe, and the programmers -are encouraged to always import this module when using the mdo-notation.) +It is enabled with the flag -XRecursiveDo, which is in turn implied by +-fglasgow-exts. + + + +Unlike ordinary do-notation, but like let and where bindings, +name shadowing is not allowed; that is, all the names bound in a single mdo must +be distinct (Section 3.3 of the paper). -As with other extensions, ghc should be given the flag -fglasgow-exts +Variables bound by a let statement in an mdo +are monomorphic in the mdo (Section 3.1 of the paper). However +GHC breaks the mdo into segments to enhance polymorphism, +and improve termination (Section 3.2 of the paper). @@ -834,7 +875,7 @@ This name is not supported by GHC. hierarchy. It completely defeats that purpose if the literal "1" means "Prelude.fromInteger 1", which is what the Haskell Report specifies. - So the flag causes + So the flag causes the following pieces of built-in syntax to refer to whatever is in scope, not the Prelude versions: @@ -905,18 +946,88 @@ fromInteger :: Integer -> Bool -> Bool you should be all right. - + +Postfix operators - - -Type system extensions + +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. + + + +Record field disambiguation + +In record construction and record pattern matching +it is entirely unambiguous which field is referred to, even if there are two different +data types in scope with a common field name. For example: + +module M where + data S = MkS { x :: Int, y :: Bool } - -Data types and type synonyms +module Foo where + import M + + data T = MkT { x :: Int } + + ok1 (MkS { x = n }) = n+1 -- Unambiguous + + ok2 n = MkT { x = n+1 } -- Unambiguous + + bad1 k = k { x = 3 } -- Ambiguous + bad2 k = x k -- Ambiguous + +Even though there are two x's in scope, +it is clear that the x in the pattern in the +definition of ok1 can only mean the field +x from type S. Similarly for +the function ok2. However, in the record update +in bad1 and the record selection in bad2 +it is not clear which of the two types is intended. + + +Haskell 98 regards all four as ambiguous, but with the + flag, GHC will accept +the former two. The rules are precisely the same as those for instance +declarations in Haskell 98, where the method names on the left-hand side +of the method bindings in an instance declaration refer unambiguously +to the method of that class (provided they are in scope at all), even +if there are other variables in scope with the same name. +This reduces the clutter of qualified names when you import two +records from different modules that use the same field name. + + + + + + + +Extensions to data types and type synonyms - + Data types with no constructors With the flag, GHC lets you declare @@ -930,13 +1041,13 @@ a data type with no constructors. For example: Syntactically, the declaration lacks the "= constrs" part. The type can be parameterised over types of any kind, but if the kind is not * then an explicit kind annotation must be used -(see ). +(see ). Such data types have only one value, namely bottom. Nevertheless, they can be useful when defining "phantom types". - + - + Infix type constructors, classes, and type variables @@ -1003,9 +1114,9 @@ to be written infix, very much like expressions. More specifically: - + - + Liberalised type synonyms @@ -1095,10 +1206,10 @@ this will be rejected: because GHC does not allow unboxed tuples on the left of a function arrow. - + - + Existentially quantified data constructors @@ -1192,7 +1303,7 @@ that collection of packages in a uniform manner. You can express quite a bit of object-oriented-like programming this way. - + Why existential? @@ -1215,9 +1326,9 @@ But Haskell programmers can safely think of the ordinary adding a new existential quantification construct. - + - + Type classes @@ -1277,9 +1388,9 @@ Notice the way that the syntax fits smoothly with that used for universal quantification earlier. - + - + Record Constructors @@ -1296,7 +1407,7 @@ data Counter a = forall self. NewCounter 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 +_inc or _display 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 @@ -1331,20 +1442,6 @@ main = do 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: @@ -1356,10 +1453,10 @@ setTag obj t = obj{ tag = t } - + - + Restrictions @@ -1485,7 +1582,7 @@ are convincing reasons to change it. You can't use deriving to define instances of a data type with existentially quantified data constructors. -Reason: in most cases it would not make sense. For example:# +Reason: in most cases it would not make sense. For example:; data T = forall a. MkT [a] deriving( Eq ) @@ -1510,185 +1607,797 @@ declarations. Define your own instances! - - + + +Declaring data types with explicit constructor signatures - -Class declarations - - -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). - - -All the extensions are enabled by the flag. - - - -Multi-parameter type classes - -Multi-parameter type classes are permitted. For example: - - +GHC allows you to declare an algebraic data type by +giving the type signatures of constructors explicitly. For example: - class Collection c a where - union :: c a -> c a -> c a - ...etc. + data Maybe a where + Nothing :: Maybe a + Just :: a -> Maybe a - - - - - -The superclasses of 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: - - +The form is called a "GADT-style declaration" +because Generalised Algebraic Data Types, described in , +can only be declared using this form. +Notice that GADT-style syntax generalises existential types (). +For example, these two declarations are equivalent: - class Functor (m k) => FiniteMap m k where - ... - - class (Monad m, Monad (t m)) => Transform t m where - lift :: m a -> (t m) a + data Foo = forall a. MkFoo a (a -> Bool) + data Foo' where { MKFoo :: a -> (a->Bool) -> Foo' } - - - -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: - - +Any data type that can be declared in standard Haskell-98 syntax +can also be declared using GADT-style syntax. +The choice is largely stylistic, but GADT-style declarations differ in one important respect: +they treat class constraints on the data constructors differently. +Specifically, if the constructor is given a type-class context, that +context is made available by pattern matching. For example: - class C a where { - op :: D b => a -> b -> b - } - - class C a => D a where { ... } - + data Set a where + MkSet :: Eq a => [a] -> Set a + makeSet :: Eq a => [a] -> Set a + makeSet xs = MkSet (nub xs) -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.) + insert :: a -> Set a -> Set a + insert a (MkSet as) | a `elem` as = MkSet as + | otherwise = MkSet (a:as) + +A use of MkSet as a constructor (e.g. in the definition of makeSet) +gives rise to a (Eq a) +constraint, as you would expect. The new feature is that pattern-matching on MkSet +(as in the definition of insert) makes available an (Eq a) +context. In implementation terms, the MkSet constructor has a hidden field that stores +the (Eq a) dictionary that is passed to MkSet; so +when pattern-matching that dictionary becomes available for the right-hand side of the match. +In the example, the equality dictionary is used to satisfy the equality constraint +generated by the call to elem, so that the type of +insert itself has no Eq constraint. - - - - - - -Class method types - - -Haskell 98 prohibits class method types to mention constraints on the -class type variable, thus: +This behaviour contrasts with Haskell 98's peculiar treament of +contexts on a data type declaration (Section 4.2.1 of the Haskell 98 Report). +In Haskell 98 the defintion - class Seq s a where - fromList :: [a] -> s a - elem :: Eq a => a -> s a -> Bool + data Eq a => Set' a = MkSet' [a] -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. - - - - - - - -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, -. - +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. -Functional dependencies are introduced by a vertical bar in the syntax of a -class declaration; e.g. +For example, a possible application of GHC's behaviour is to reify dictionaries: - class (Monad m) => MonadState s m | m -> s where ... + data NumInst a where + MkNumInst :: Num a => NumInst a - class Foo a b c | a b -> c where ... + intInst :: NumInst Int + intInst = MkNumInst + + plus :: NumInst a -> a -> a -> a + plus MkNumInst p q = p + q -There should be more documentation, but there isn't (yet). Yell if you need it. +Here, a value of type NumInst a is equivalent +to an explicit (Num a) dictionary. -Rules for functional dependencies -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: +The rest of this section gives further details about GADT-style data +type declarations. + + + +The result type of each data constructor must begin with the type constructor being defined. +If the result type of all constructors +has the form T a1 ... an, where a1 ... an +are distinct type variables, then the data type is ordinary; +otherwise is a generalised data type (). + + +The type signature of +each constructor is independent, and is implicitly universally quantified as usual. +Different constructors may have different universally-quantified type variables +and different type-class constraints. +For example, this is fine: - class Coll s a where - empty :: s - insert :: s -> a -> s + data T a where + T1 :: Eq b => b -> T b + T2 :: (Show c, Ix c) => c -> [c] -> T c + -is not OK, because the type of empty doesn't mention -a. Functional dependencies can make the type variable -reachable: + +Unlike a Haskell-98-style +data type declaration, the type variable(s) in the "data Set a where" header +have no scope. Indeed, one can write a kind signature instead: - class Coll s a | s -> a where - empty :: s - insert :: s -> a -> s + data Set :: * -> * where ... + +or even a mixture of the two: + + data Foo a :: (* -> *) -> * where ... + +The type variables (if given) may be explicitly kinded, so we could also write the header for Foo +like this: + + data Foo a (b :: * -> *) where ... + -Alternatively Coll might be rewritten + +You can use strictness annotations, in the obvious places +in the constructor type: - class Coll s a where - empty :: s a - insert :: s a -> a -> s a + data Term a where + Lit :: !Int -> Term Int + If :: Term Bool -> !(Term a) -> !(Term a) -> Term a + Pair :: Term a -> Term b -> Term (a,b) + + +You can use a deriving clause on a GADT-style data type +declaration. For example, these two declarations are equivalent + + data Maybe1 a where { + Nothing1 :: Maybe1 a ; + Just1 :: a -> Maybe1 a + } deriving( Eq, Ord ) -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: + data Maybe2 a = Nothing2 | Just2 a + deriving( Eq, Ord ) + + + +You can use record syntax on a GADT-style data type declaration: - class CollE s where - empty :: s - - class CollE s => Coll s a where - insert :: s -> a -> s + data Person where + Adult { name :: String, children :: [Person] } :: Person + Child { name :: String } :: Person +As usual, for every constructor that has a field f, the type of +field f must be the same (modulo alpha conversion). - - + +At the moment, record updates are not yet possible with GADT-style declarations, +so support is limited to record construction, selection and pattern matching. +For exmaple + + aPerson = Adult { name = "Fred", children = [] } - -Background on functional dependencies + shortName :: Person -> Bool + hasChildren (Adult { children = kids }) = not (null kids) + hasChildren (Child {}) = False + + -The following description of the motivation and use of functional dependencies is taken -from the Hugs user manual, reproduced here (with minor changes) by kind -permission of Mark Jones. - - -Consider the following class, intended as part of a -library for collection types: + +As in the case of existentials declared using the Haskell-98-like record syntax +(), +record-selector functions are generated only for those fields that have well-typed +selectors. +Here is the example of that section, in GADT-style syntax: + +data Counter a where + NewCounter { _this :: self + , _inc :: self -> self + , _display :: self -> IO () + , tag :: a + } + :: Counter a + +As before, only one selector function is generated here, that for tag. +Nevertheless, you can still use all the field names in pattern matching and record construction. + + + + + +Generalised Algebraic Data Types (GADTs) + +Generalised Algebraic Data Types generalise ordinary algebraic data types +by allowing constructors to have richer return types. Here is an example: + + data Term a where + Lit :: Int -> Term Int + Succ :: Term Int -> Term Int + IsZero :: Term Int -> Term Bool + If :: Term Bool -> Term a -> Term a -> Term a + Pair :: Term a -> Term b -> Term (a,b) + +Notice that the return type of the constructors is not always Term a, as is the +case with ordinary data types. This generality allows us to +write a well-typed eval function +for these Terms: + + eval :: Term a -> a + eval (Lit i) = i + eval (Succ t) = 1 + eval t + 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 e1, eval e2) + +The key point about GADTs is that pattern matching causes type refinement. +For example, in the right hand side of the equation + + eval :: Term a -> a + eval (Lit i) = ... + +the type a is refined to Int. That's the whole point! +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 +unification-based type inference for GADTs, +(ICFP 2006). +The general principle is this: type refinement is only carried out +based on user-supplied type annotations. +So if no type signature is supplied for eval, no type refinement happens, +and lots of obscure error messages will +occur. However, the refinement is quite general. For example, if we had: + + eval :: Term a -> a -> a + eval (Lit i) j = i+j + +the pattern match causes the type a to be refined to Int (because of the type +of the constructor Lit), and that refinement also applies to the type of j, and +the result type of the case expression. Hence the addition i+j is legal. + + +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. The extension is enabled with +. + + +A GADT can only be declared using GADT-style syntax (); +the old Haskell-98 syntax for data declarations always declares an ordinary data type. +The result type of each constructor must begin with the type constructor being defined, +but for a GADT the arguments to the type constructor can be arbitrary monotypes. +For example, in the Term data +type above, the type of each constructor must end with Term ty, but +the ty may not be a type variable (e.g. the Lit +constructor). + + + +You cannot use a deriving clause for a GADT; only for +an ordianary data type. + + + +As mentioned in , record syntax is supported. +For example: + + 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 + +However, for GADTs there is the following additional constraint: +every constructor that has a field f must have +the same result type (modulo alpha conversion) +Hence, in the above 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 + + + + + + + + + + + + +Extensions to the "deriving" mechanism + + +Inferred context for deriving clauses + + +The Haskell Report is vague about exactly when a deriving clause is +legal. For example: + + data T0 f a = MkT0 a deriving( Eq ) + data T1 f a = MkT1 (f a) deriving( Eq ) + data T2 f a = MkT2 (f (f a)) deriving( Eq ) + +The natural generated Eq code would result in these instance declarations: + + instance Eq a => Eq (T0 f a) where ... + instance Eq (f a) => Eq (T1 f a) where ... + instance Eq (f (f a)) => Eq (T2 f a) where ... + +The first of these is obviously fine. The second is still fine, although less obviously. +The third is not Haskell 98, and risks losing termination of instances. + + +GHC takes a conservative position: it accepts the first two, but not the third. The rule is this: +each constraint in the inferred instance context must consist only of type variables, +with no repititions. + + +This rule is applied regardless of flags. If you want a more exotic context, you can write +it yourself, using the standalone deriving mechanism. + + + + +Stand-alone deriving declarations + + +GHC now allows stand-alone deriving declarations, enabled by -XStandaloneDeriving: + + data Foo a = Bar a | Baz String + + deriving instance Eq a => Eq (Foo a) + +The syntax is identical to that of an ordinary instance declaration apart from (a) the keyword +deriving, and (b) the absence of the where part. +You must supply a context (in the example the context is (Eq a)), +exactly as you would in an ordinary instance declaration. +(In contrast the context is inferred in a deriving clause +attached to a data type declaration.) These deriving instance +rules obey the same rules concerning form and termination as ordinary instance declarations, +controlled by the same flags; see . + +The stand-alone syntax is generalised for newtypes in exactly the same +way that ordinary deriving clauses are generalised (). +For example: + + newtype Foo a = MkFoo (State Int a) + + deriving instance MonadState Int Foo + +GHC always treats the last parameter of the instance +(Foo in this exmample) as the type whose instance is being derived. + + + + + + +Deriving clause for classes <literal>Typeable</literal> and <literal>Data</literal> + + +Haskell 98 allows the programmer to add "deriving( Eq, Ord )" to a data type +declaration, to generate a standard instance declaration for classes specified in the deriving clause. +In Haskell 98, the only classes that may appear in the deriving clause are the standard +classes Eq, Ord, +Enum, Ix, Bounded, Read, and Show. + + +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.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. + + + + +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, +using the flag , +so one can write + + newtype Dollars = Dollars Int deriving (Eq,Show,Num) + + +and the implementation uses the same Num dictionary +for Dollars as for Int. Notionally, the compiler +derives an instance declaration of the form + + + instance Num Int => Num Dollars + + +which just adds or removes the newtype constructor according to the type. + + + +We can also derive instances of constructor classes in a similar +way. For example, suppose we have implemented state and failure monad +transformers, such that + + + instance Monad m => Monad (State s m) + instance Monad m => Monad (Failure m) + +In Haskell 98, we can define a parsing monad by + + type Parser tok m a = State [tok] (Failure m) a + + +which is automatically a monad thanks to the instance declarations +above. With the extension, we can make the parser type abstract, +without needing to write an instance of class Monad, via + + + newtype Parser tok m a = Parser (State [tok] (Failure m) a) + deriving Monad + +In this case the derived instance declaration is of the form + + instance Monad (State [tok] (Failure m)) => Monad (Parser tok m) + + +Notice that, since Monad is a constructor class, the +instance is a partial application of the new type, not the +entire left hand side. We can imagine that the type declaration is +"eta-converted" to generate the context of the instance +declaration. + + + +We can even derive instances of multi-parameter classes, provided the +newtype is the last class parameter. In this case, a ``partial +application'' of the class appears in the deriving +clause. For example, given the class + + + class StateMonad s m | m -> s where ... + instance Monad m => StateMonad s (State s m) where ... + +then we can derive an instance of StateMonad for Parsers by + + newtype Parser tok m a = Parser (State [tok] (Failure m) a) + deriving (Monad, StateMonad [tok]) + + +The derived instance is obtained by completing the application of the +class to the new type: + + + instance StateMonad [tok] (State [tok] (Failure m)) => + StateMonad [tok] (Parser tok m) + + + + +As a result of this extension, all derived instances in newtype + declarations are treated uniformly (and implemented just by reusing +the dictionary for the representation type), except +Show and Read, which really behave differently for +the newtype and its representation. + + + + A more precise specification + +Derived instance declarations are constructed as follows. Consider the +declaration (after expansion of any type synonyms) + + + newtype T v1...vn = T' (t vk+1...vn) deriving (c1...cm) + + +where + + + 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 k is chosen so that ci (T v1...vk) is well-kinded. + + + 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, + Typeable, or Data. These classes + should not "look through" the type or its constructor. You can still + derive these classes for a newtype, but it happens in the usual way, not + via this new mechanism. + + +Then, for each ci, the derived instance +declaration is: + + instance ci t => ci (T v1...vk) + +As an example which does not work, consider + + newtype NonMonad m s = NonMonad (State s m s) deriving Monad + +Here we cannot derive the instance + + instance Monad (State s m) => Monad (NonMonad m) + + +because the type variable s occurs in State s m, +and so cannot be "eta-converted" away. It is a good thing that this +deriving clause is rejected, because NonMonad m is +not, in fact, a monad --- for the same reason. Try defining +>>= with the correct type: you won't be able to. + + + +Notice also that the order of class parameters becomes +important, since we can only derive instances for the last one. If the +StateMonad class above were instead defined as + + + class StateMonad m s | m -> s where ... + + +then we would not have been able to derive an instance for the +Parser type above. We hypothesise that multi-parameter +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.) + + + + + + + + +Class and instances declarations + + +Class declarations + + +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). + + +All the extensions are enabled by the flag. + + + +Multi-parameter type classes + +Multi-parameter type classes are permitted. For example: + + + + class Collection c a where + union :: c a -> c a -> c a + ...etc. + + + + + + +The superclasses of 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: + + + + class Functor (m k) => FiniteMap m k where + ... + + class (Monad m, Monad (t m)) => Transform t m where + lift :: m a -> (t m) a + + + + + +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 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.) + + + + + + + +Class method types + + +Haskell 98 prohibits class method types to mention constraints on the +class type variable, thus: + + class Seq s a where + fromList :: [a] -> s a + elem :: Eq a => a -> s a -> Bool + +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. + + + + + + + +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. + + +Rules for functional dependencies + +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: + + + class Coll s a where + empty :: s + insert :: s -> a -> s + + +is not OK, because the type of empty doesn't mention +a. Functional dependencies can make the type variable +reachable: + + class Coll s a | s -> a where + empty :: s + insert :: s -> a -> s + + +Alternatively Coll might be rewritten + + + class Coll s a where + empty :: s a + insert :: s a -> a -> s a + + + +which makes the connection between the type of a collection of +a's (namely (s a)) and the element type a. +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 + + + + + + +Background on functional dependencies + +The following description of the motivation and use of functional dependencies is taken +from the Hugs user manual, reproduced here (with minor changes) by kind +permission of Mark Jones. + + +Consider the following class, intended as part of a +library for collection types: class Collects e ce where empty :: ce @@ -1957,7 +2666,7 @@ the context and head of the instance declaration can each consist of arbitrary following rules: -For each assertion in the context: +The Paterson Conditions: for each assertion in the context No type variable has more occurrences in the assertion than in the head The assertion has fewer constructors and variables (taken together @@ -1965,7 +2674,7 @@ For each assertion in the context: -The coverage condition. For each functional dependency, +The Coverage Condition. For each functional dependency, tvsleft -> tvsright, of the class, every type variable in @@ -1977,11 +2686,15 @@ corresponding type in the instance declaration. These restrictions ensure that context reduction terminates: each reduction step makes the problem smaller by at least one -constructor. For example, the following would make the type checker -loop if it wasn't excluded: - - instance C a => C a where ... - +constructor. Both the Paterson Conditions and the Coverage Condition are lifted +if you give the +flag (). +You can find lots of background material about the reason for these +restrictions in the paper +Understanding functional dependencies via Constraint Handling Rules. + + For example, these are OK: instance C Int [a] -- Multiple parameters @@ -2033,11 +2746,6 @@ something more specific does not: op = ... -- Default -You can find lots of background material about the reason for these -restrictions in the paper -Understanding functional dependencies via Constraint Handling Rules. - @@ -2100,10 +2808,10 @@ makes instance inference go into a loop, because it requires the constraint Nevertheless, GHC allows you to experiment 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 +the experimental flag +-XUndecidableInstances, +both the Paterson Conditions and the Coverage Condition +(described in ) are lifted. 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. @@ -2118,11 +2826,11 @@ with N. In general, GHC requires that that it be unambiguous which instance declaration should be used to resolve a type-class constraint. This behaviour -can be modified by two flags: --fallow-overlapping-instances +can be modified by two flags: +-XOverlappingInstances -and --fallow-incoherent-instances +and +-XIncoherentInstances , as this section discusses. Both these flags are dynamic flags, and can be set on a per-module basis, using an OPTIONS_GHC pragma if desired (). @@ -2150,7 +2858,7 @@ particular constraint matches more than one. -The flag instructs GHC to allow +The flag instructs GHC to allow more than one instance to match, provided there is a most specific one. For example, the constraint C Int [Int] matches instances (A), (C) and (D), but the last is more specific, and hence is chosen. If there is no @@ -2167,30 +2875,45 @@ Suppose that from the RHS of f we get the constraint GHC does not commit to instance (C), because in a particular call of f, b might be instantiate to Int, in which case instance (D) would be more specific still. -So GHC rejects the program. If you add the flag , +So GHC rejects the program. +(If you add the flag , GHC will instead pick (C), without complaining about -the problem of subsequent instantiations. +the problem of subsequent instantiations.) + + +Notice that we gave a type signature to f, so GHC had to +check that f has the specified type. +Suppose instead we do not give a type signature, asking GHC to infer +it instead. In this case, GHC will refrain from +simplifying the constraint C Int [Int] (for the same reason +as before) but, rather than rejecting the program, it will infer the type + + f :: C Int b => [b] -> [b] + +That postpones the question of which instance to pick to the +call site for f +by which time more is known about the type b. 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 +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 +. The flag setting for the more-specific instance does not matter. -Suppose an instance declaration does not matche the constraint being looked up, but +Suppose an instance declaration does not match 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?" +, GHC will skip the "does-it-unify?" check for that declaration. @@ -2199,18 +2922,18 @@ overlapping instances without the library client having to know. If an instance declaration is compiled without -, +, then that instance can never be overlapped. This could perhaps be inconvenient. Perhaps the rule should instead say that the overlapping instance declaration should be compiled in this way, rather than the overlapped one. Perhaps overlap at a usage site should be permitted regardless of how the instance declarations -are compiled, if the flag is +are compiled, if the flag is used at the usage site. (Mind you, the exact usage site can occasionally be hard to pin down.) We are interested to receive feedback on these points. -The flag implies the - flag, but not vice versa. +The flag implies the + flag, but not vice versa. @@ -2259,6 +2982,86 @@ reversed, but it makes sense to me. + +Overloaded string literals + + + +GHC supports overloaded string literals. Normally a +string literal has type String, but with overloaded string +literals enabled (with -XOverloadedStrings) + a string literal has type (IsString a) => a. + + +This means that the usual string syntax can be used, e.g., for packed strings +and other variations of string like types. String literals behave very much +like integer literals, i.e., they can be used in both expressions and patterns. +If used in a pattern the literal with be replaced by an equality test, in the same +way as an integer literal is. + + +The class IsString is defined as: + +class IsString a where + fromString :: String -> a + +The only predefined instance is the obvious one to make strings work as usual: + +instance IsString [Char] where + fromString cs = cs + +The class IsString is not in scope by default. If you want to mention +it explicitly (for exmaple, to give an instance declaration for it), you can import it +from module GHC.Exts. + + +Haskell's defaulting mechanism is extended to cover string literals, when is specified. +Specifically: + + +Each type in a default declaration must be an +instance of Num or of IsString. + + + +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. + + + + +A small example: + +module Main where + +import GHC.Exts( IsString(..) ) + +newtype MyString = MyString String deriving (Eq, Show) +instance IsString MyString where + fromString = MyString + +greet :: MyString -> MyString +greet "hello" = "world" +greet other = other + +main = do + print $ greet "hello" + print $ greet "fool" + + + +Note that deriving Eq is necessary for the pattern matching +to work since it gets translated into an equality comparison. + + + + + + +Other type system extensions + Type signatures @@ -2375,54 +3178,6 @@ territory free in case we need it later. - -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 - - - -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 - - - @@ -2441,7 +3196,7 @@ Boston, Jan 2000. due to Jeff Lewis.) Implicit parameter support is enabled with the option -. +. A variable is called dynamically bound when it is bound by the calling @@ -2831,7 +3586,7 @@ and you'd be right. That is why they are an experimental feature. ================ END OF Linear Implicit Parameters commented out --> - + Explicitly-kinded quantification @@ -2925,6 +3680,8 @@ For example, all the following types are legal: g2 :: (forall a. Eq a => [a] -> a -> Bool) -> Int -> Int f3 :: ((forall a. a->a) -> Int) -> Bool -> Bool + + f4 :: Int -> (forall a. a -> a) Here, f1 and g1 are rank-1 types, and can be written in standard Haskell (e.g. f1 :: a->b->a). @@ -2947,22 +3704,14 @@ that restriction has now been lifted.) In particular, a forall-type (also called a "type scheme"), including an operational type class context, is legal: - On the left of a function arrow - On the right of a function arrow (see ) + On the left or right (see f4, for example) +of a function arrow As the argument of a constructor, or type of a field, in a data type declaration. For example, any of the f1,f2,f3,g1,g2 above would be valid field type signatures. As the type of an implicit parameter In a pattern type signature (see ) -There is one place you cannot put a forall: -you cannot instantiate a type variable with a forall-type. So you cannot -make a forall-type the argument of a type constructor. So these types are illegal: - - x1 :: [forall a. a->a] - x2 :: (forall a. a->a, Int) - x3 :: Maybe (forall a. a->a) - Of course forall becomes a keyword; you can't use forall as a type variable any more! @@ -3269,6 +4018,7 @@ 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 +4070,23 @@ 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 +4095,7 @@ 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) @@ -3402,283 +4169,51 @@ examples would parse quite differently with parentheses: {- f assumes that 'a' is already in scope -} f x (y :: [a]) = [x,y,x] - g = \ (x :: [Int]) -> [3,4] - - h :: forall a. [a] -> a - h xs = case xs of - ((y:ys) :: a) -> y - -Now the signature is on the pattern; and -h would certainly be ill-typed (since the pattern -(y:ys) cannot have the type a. - -Second, to avoid ambiguity, the type after the “::” in a result -pattern signature on a lambda or case must be atomic (i.e. a single -token or a parenthesised type of some sort). To see why, -consider how one would parse this: - - \ x :: a -> b -> x - - - - - --> - - -Class and instance declarations - - -The type variables in the head of a class or instance declaration -scope over the methods defined in the where part. For example: - - - - class C a where - op :: [a] -> a - - op xs = let ys::[a] - ys = reverse xs - in - head ys - - - - - - - -Deriving clause for classes <literal>Typeable</literal> and <literal>Data</literal> - - -Haskell 98 allows the programmer to add "deriving( Eq, Ord )" to a data type -declaration, to generate a standard instance declaration for classes specified in the deriving clause. -In Haskell 98, the only classes that may appear in the deriving clause are the standard -classes Eq, Ord, -Enum, Ix, Bounded, Read, and Show. - - -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.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. - - - - -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) - - -and the implementation uses the same Num dictionary -for Dollars as for Int. Notionally, the compiler -derives an instance declaration of the form - - - instance Num Int => Num Dollars - - -which just adds or removes the newtype constructor according to the type. - - - -We can also derive instances of constructor classes in a similar -way. For example, suppose we have implemented state and failure monad -transformers, such that - - - instance Monad m => Monad (State s m) - instance Monad m => Monad (Failure m) - -In Haskell 98, we can define a parsing monad by - - type Parser tok m a = State [tok] (Failure m) a - - -which is automatically a monad thanks to the instance declarations -above. With the extension, we can make the parser type abstract, -without needing to write an instance of class Monad, via - - - newtype Parser tok m a = Parser (State [tok] (Failure m) a) - deriving Monad - -In this case the derived instance declaration is of the form - - instance Monad (State [tok] (Failure m)) => Monad (Parser tok m) - - -Notice that, since Monad is a constructor class, the -instance is a partial application of the new type, not the -entire left hand side. We can imagine that the type declaration is -``eta-converted'' to generate the context of the instance -declaration. - - - -We can even derive instances of multi-parameter classes, provided the -newtype is the last class parameter. In this case, a ``partial -application'' of the class appears in the deriving -clause. For example, given the class - - - class StateMonad s m | m -> s where ... - instance Monad m => StateMonad s (State s m) where ... - -then we can derive an instance of StateMonad for Parsers by - - newtype Parser tok m a = Parser (State [tok] (Failure m) a) - deriving (Monad, StateMonad [tok]) - - -The derived instance is obtained by completing the application of the -class to the new type: - - - instance StateMonad [tok] (State [tok] (Failure m)) => - StateMonad [tok] (Parser tok m) + g = \ (x :: [Int]) -> [3,4] + + h :: forall a. [a] -> a + h xs = case xs of + ((y:ys) :: a) -> y - - +Now the signature is on the pattern; and +h would certainly be ill-typed (since the pattern +(y:ys) cannot have the type a. -As a result of this extension, all derived instances in newtype - declarations are treated uniformly (and implemented just by reusing -the dictionary for the representation type), except -Show and Read, which really behave differently for -the newtype and its representation. +Second, to avoid ambiguity, the type after the “::” in a result +pattern signature on a lambda or case must be atomic (i.e. a single +token or a parenthesised type of some sort). To see why, +consider how one would parse this: + + \ x :: a -> b -> x + - A more precise specification - -Derived instance declarations are constructed as follows. Consider the -declaration (after expansion of any type synonyms) - - - newtype T v1...vn = T' (t vk+1...vn) deriving (c1...cm) - + --> -where - - - The type t is an arbitrary type - - - The vk+1...vn are type variables which do not occur in - t, 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. - - - None of the ci is Read, Show, - Typeable, or Data. These classes - should not "look through" the type or its constructor. You can still - derive these classes for a newtype, but it happens in the usual way, not - via this new mechanism. - - -Then, for each ci, the derived instance -declaration is: - - instance ci (t vk+1...v) => ci (T v1...vp) - -where p is chosen so that T v1...vp is of the -right kind for the last parameter of class Ci. - + +Class and instance declarations -As an example which does not work, consider - - newtype NonMonad m s = NonMonad (State s m s) deriving Monad - -Here we cannot derive the instance - - instance Monad (State s m) => Monad (NonMonad m) - +The type variables in the head of a class or instance declaration +scope over the methods defined in the where part. For example: -because the type variable s occurs in State s m, -and so cannot be "eta-converted" away. It is a good thing that this -deriving clause is rejected, because NonMonad m is -not, in fact, a monad --- for the same reason. Try defining ->>= with the correct type: you won't be able to. - - -Notice also that the order of class parameters becomes -important, since we can only derive instances for the last one. If the -StateMonad class above were instead defined as + + class C a where + op :: [a] -> a - - class StateMonad m s | m -> s where ... + op xs = let ys::[a] + ys = reverse xs + in + head ys - -then we would not have been able to derive an instance for the -Parser type above. We hypothesise that multi-parameter -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 @@ -3699,7 +4234,7 @@ and all others are monomorphic until the group is generalised Following a suggestion of Mark Jones, in his paper Typing Haskell in Haskell, -GHC implements a more general scheme. If is +GHC implements a more general scheme. If is specified: the dependency analysis ignores references to variables that have an explicit type signature. @@ -3728,7 +4263,7 @@ Now, the defintion for f is typechecked, with this type for 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: @@ -3742,181 +4277,36 @@ pattern binding must have the same context. For example, this is fine: - - - - - - -Generalised Algebraic Data Types + +Type families + -Generalised Algebraic Data Types (GADTs) generalise ordinary algebraic data types by allowing you -to give the type signatures of constructors explicitly. For example: - - data Term a where - Lit :: Int -> Term Int - Succ :: Term Int -> Term Int - IsZero :: Term Int -> Term Bool - If :: Term Bool -> Term a -> Term a -> Term a - Pair :: Term a -> Term b -> Term (a,b) - -Notice that the return type of the constructors is not always Term a, as is the -case with ordinary vanilla data types. Now we can write a well-typed eval function -for these Terms: - - eval :: Term a -> a - eval (Lit i) = i - eval (Succ t) = 1 + eval t - 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 e1, eval e2) - -These and many other examples are given in papers by Hongwei Xi, and Tim Sheard. + +GHC supports the definition of type families indexed by types. They may be +seen as an extension of Haskell 98's class-based overloading of values to +types. When type families are declared in classes, they are also known as +associated types. -The rest of this section outlines the extensions to GHC that support GADTs. -It is far from comprehensive, but the design closely follows that described in -the paper Simple -unification-based type inference for GADTs, -which appeared in ICFP 2006. - - - Data type declarations have a 'where' form, as exemplified above. The type signature of -each constructor is independent, and is implicitly universally quantified as usual. Unlike a normal -Haskell data type declaration, the type variable(s) in the "data Term a where" header -have no scope. Indeed, one can write a kind signature instead: - - data Term :: * -> * where ... - -or even a mixture of the two: - - data Foo a :: (* -> *) -> * where ... - -The type variables (if given) may be explicitly kinded, so we could also write the header for Foo -like this: - - data Foo a (b :: * -> *) where ... - - - - -There are no restrictions on the type of the data constructor, except that the result -type must begin with the type constructor being defined. For example, in the Term data -type above, the type of each constructor must end with ... -> Term .... - - - -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 - Lit :: !Int -> Term Int - If :: Term Bool -> !(Term a) -> !(Term a) -> Term a - Pair :: Term a -> Term b -> Term (a,b) - - - - -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 - eval (Lit i) = ... - -the type a is refined to Int. (That's the whole point!) -A precise specification of the type rules is beyond what this user manual aspires to, but there is a paper -about the ideas: "Wobbly types: practical type inference for generalised algebraic data types", on Simon PJ's home page. - - The general principle is this: type refinement is only carried out based on user-supplied type annotations. -So if no type signature is supplied for eval, no type refinement happens, and lots of obscure error messages will -occur. However, the refinement is quite general. For example, if we had: - - eval :: Term a -> a -> a - eval (Lit i) j = i+j - -the pattern match causes the type a to be refined to Int (because of the type -of the constructor Lit, and that refinement also applies to the type of j, and -the result type of the case expression. Hence the addition i+j is legal. +There are two forms of type families: data families and type synonym families. +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 +wiki page on type families. The material will be moved to this user's +guide when it has stabilised. - - + +Type families are enabled by the flag . -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 } - - - - + + + + @@ -3957,9 +4347,10 @@ Tim Sheard is going to expand it.) Template Haskell has the following new syntactic constructions. You need to use the flag - + + to switch these syntactic extensions on - ( is no longer implied by + ( is no longer implied by ). @@ -3991,7 +4382,7 @@ 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]. - [Planned, but not implemented yet.] [t| ... |], where the "..." is a type; + [t| ... |], where the "..." is a type; the quotation has type Type. @@ -4025,6 +4416,14 @@ Tim Sheard is going to expand it.) (It would make sense to do so, but it's hard to implement.) + + Furthermore, you can only run a function at compile time if it is imported + from another module that is not part of a mutually-recursive group of modules + that includes the module currently being compiled. For example, when compiling module A, + you can only run Template Haskell functions imported from B if B does not import A (directly or indirectly). + The reason should be clear: to run B we must compile and run A, but we are currently type-checking A. + + The flag -ddump-splices shows the expansion of all top-level splices as they happen. @@ -4097,7 +4496,7 @@ pr s = gen (parse s) Now run the compiler (here we are a Cygwin prompt on Windows): -$ ghc --make -fth main.hs -o main.exe +$ ghc --make -XTemplateHaskell main.hs -o main.exe Run "main.exe" and here is your output: @@ -4186,7 +4585,7 @@ Palgrave, 2003. and the arrows web page at http://www.haskell.org/arrows/. -With the flag, GHC supports the arrow +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. @@ -4644,7 +5043,7 @@ Because the preprocessor targets Haskell (rather than Core), - + Bang patterns <indexterm><primary>Bang patterns</primary></indexterm> @@ -4656,10 +5055,10 @@ prime feature description contains more discussion and examples than the material below. -Bang patterns are enabled by the flag . +Bang patterns are enabled by the flag . - + Informal description of bang patterns @@ -4714,7 +5113,7 @@ is part of the syntax of let bindings. - + Syntax and semantics @@ -4729,7 +5128,7 @@ f !x = 3 Is this a definition of the infix function "(!)", or of the "f" with a bang pattern? GHC resolves this -ambiguity inf favour of the latter. If you want to define +ambiguity in favour of the latter. If you want to define (!) with bang-patterns enabled, you have to do so using prefix notation: @@ -4788,7 +5187,7 @@ a module. - + Assertions <indexterm><primary>Assertions</primary></indexterm> @@ -5757,12 +6156,6 @@ The following are good consumers: - length - - - - - ++ (on its first argument) @@ -6030,7 +6423,7 @@ r) GHCziBase.ZMZN GHCziBase.Char -> GHCziBase.ZMZN GHCziBase.Cha r) -> tpl2}) - (%note "foo" + (%note "bar" eta); @@ -6048,84 +6441,10 @@ r) -> Special built-in functions -GHC has a few built-in funcions with special behaviour, -described in this section. All are exported by -GHC.Exts. - - The <literal>inline</literal> function - -The inline function is somewhat experimental. - - inline :: a -> a - -The call (inline f) arranges that f -is inlined, regardless of its size. More precisely, the call -(inline f) rewrites to the right-hand side of f's -definition. -This allows the programmer to control inlining from -a particular call site -rather than the definition site of the function -(c.f. INLINE pragmas ). - - -This inlining occurs regardless of the argument to the call -or the size of f's definition; it is unconditional. -The main caveat is that f's definition must be -visible to the compiler. That is, f must be -let-bound in the current scope. -If no inlining takes place, the inline function -expands to the identity function in Phase zero; so its use imposes -no overhead. - - If the function is defined in another -module, GHC only exposes its inlining in the interface file if the -function is sufficiently small that it might be -inlined by the automatic mechanism. There is currently no way to tell -GHC to expose arbitrarily-large functions in the interface file. (This -shortcoming is something that could be fixed, with some kind of pragma.) - - - - The <literal>lazy</literal> function - -The lazy function restrains strictness analysis a little: - - lazy :: a -> a - -The call (lazy e) means the same as e, -but lazy has a magical property so far as strictness -analysis is concerned: it is lazy in its first argument, -even though its semantics is strict. After strictness analysis has run, -calls to lazy are inlined to be the identity function. - - -This behaviour is occasionally useful when controlling evaluation order. -Notably, lazy is used in the library definition of -Control.Parallel.par: - - par :: a -> b -> b - par x y = case (par# x) of { _ -> lazy y } - -If lazy were not lazy, par would -look strict in y which would defeat the whole -purpose of par. - - - - The <literal>unsafeCoerce#</literal> function - -The function unsafeCoerce# allows you to side-step the -typechecker entirely. It has type - - unsafeCoerce# :: a -> b - -That is, it allows you to coerce any type into any other type. If you use this -function, you had better get it right, otherwise segmentation faults await. -It is generally used when you want to write a program that you know is -well-typed, but where Haskell's type system is not expressive enough to prove -that it is well typed. - - +GHC has a few built-in funcions with special behaviour. These +are now described in the module GHC.Prim +in the library documentation. @@ -6182,7 +6501,7 @@ where clause and over-ride whichever methods you please. Use the flags (to enable the extra syntax), - (to generate extra per-data-type code), + (to generate extra per-data-type code), and (to make the Generics library available. @@ -6391,21 +6710,21 @@ carried out at let and where bindings. Switching off the dreaded Monomorphism Restriction - + Haskell's monomorphism restriction (see Section 4.5.5 of the Haskell Report) can be completely switched off by -. +. Monomorphic pattern bindings - - + + As an experimental change, we are exploring the possibility of making pattern bindings monomorphic; that is, not generalised at all. @@ -6421,7 +6740,7 @@ 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.