X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=docs%2Fusers_guide%2Fglasgow_exts.xml;h=4475af4bf523653d46d60994578da9bd5b66045c;hb=500fb9ad138112e8145d9f125f8c6d013f449205;hp=6dfda6b84fbeee125d1af01fff7ed64496cfb7b4;hpb=63006863be568d5cbb21d3198d43859c743152ce;p=ghc-hetmet.git diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index 6dfda6b..4475af4 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! - + @@ -3364,7 +3372,7 @@ 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 +3492,6 @@ 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 +4063,81 @@ 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 +4249,14 @@ 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 +4876,7 @@ Because the preprocessor targets Haskell (rather than Core), - + Bang patterns <indexterm><primary>Bang patterns</primary></indexterm> @@ -4801,7 +4891,7 @@ than the material below. Bang patterns are enabled by the flag . - + Informal description of bang patterns @@ -4856,7 +4946,7 @@ is part of the syntax of let bindings. - + Syntax and semantics @@ -4930,7 +5020,7 @@ a module. - + Assertions <indexterm><primary>Assertions</primary></indexterm> @@ -5899,12 +5989,6 @@ The following are good consumers: - length - - - - - ++ (on its first argument)