X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=docs%2Fusers_guide%2Fglasgow_exts.xml;h=d9a6198788717b1d2299784935c4a47e0e0dfb50;hb=78b620de24ceb4621cead67bd642f72749b41052;hp=9e93fd73db1392c1a4f91efb40cdfa4e7a0c250b;hpb=8d3a648c4ef8b2be9f95bade7ff1516a893ea488;p=ghc-hetmet.git diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index 9e93fd7..d9a6198 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -241,6 +241,14 @@ documentation describes all the libraries that come with GHC. + + + Enables overloaded string literals (see ). + + + + Enables lexically-scoped type variables (see 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". @@ -1218,7 +1226,7 @@ that collection of packages in a uniform manner. You can express quite a bit of object-oriented-like programming this way. - + Why existential? @@ -1241,9 +1249,9 @@ But Haskell programmers can safely think of the ordinary adding a new existential quantification construct. - + - + Type classes @@ -1303,9 +1311,9 @@ Notice the way that the syntax fits smoothly with that used for universal quantification earlier. - + - + Record Constructors @@ -1322,7 +1330,7 @@ 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 @@ -1368,10 +1376,10 @@ setTag obj t = obj{ tag = t } - + - + Restrictions @@ -1522,7 +1530,7 @@ declarations. Define your own instances! - + @@ -2538,7 +2546,7 @@ 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 @@ -2546,7 +2554,7 @@ 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 @@ -2558,11 +2566,15 @@ 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 @@ -2614,11 +2626,6 @@ 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. - @@ -2683,8 +2690,8 @@ 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 +option, 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. @@ -3364,7 +3371,7 @@ and you'd be right. That is why they are an experimental feature. ================ END OF Linear Implicit Parameters commented out --> - + Explicitly-kinded quantification @@ -3484,7 +3491,6 @@ including an operational type class context, is legal: On the left or right (see f4, for example) of a function arrow - On the right of a function arrow (see ) 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. @@ -4056,6 +4062,81 @@ pattern binding must have the same context. For example, this is fine: + +Overloaded string literals + + + +GHC supports overloaded string literals. Normally a +string literal has type String, but with overloaded string +literals enabled (with -foverloaded-strings) + 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. + + + @@ -4167,6 +4248,14 @@ 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. @@ -4786,7 +4875,7 @@ Because the preprocessor targets Haskell (rather than Core), - + Bang patterns <indexterm><primary>Bang patterns</primary></indexterm> @@ -4801,7 +4890,7 @@ than the material below. Bang patterns are enabled by the flag . - + Informal description of bang patterns @@ -4856,7 +4945,7 @@ is part of the syntax of let bindings. - + Syntax and semantics @@ -4930,7 +5019,7 @@ a module. - + Assertions <indexterm><primary>Assertions</primary></indexterm>