X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=docs%2Fusers_guide%2Fglasgow_exts.xml;h=4475af4bf523653d46d60994578da9bd5b66045c;hb=7f3ce06a1a2840c52d6f5bf1bcd09cffe1f80d28;hp=0c3b07cb6c50d9e3bda756061f9b75083a8cc85a;hpb=f94f26a7cb7bb7005b88f50c4383a761fc18718d;p=ghc-hetmet.git diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index 0c3b07c..4475af4 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -964,7 +964,7 @@ 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". @@ -1226,7 +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? @@ -1249,9 +1249,9 @@ But Haskell programmers can safely think of the ordinary adding a new existential quantification construct. - + - + Type classes @@ -1311,9 +1311,9 @@ Notice the way that the syntax fits smoothly with that used for universal quantification earlier. - + - + Record Constructors @@ -1330,7 +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 @@ -1376,10 +1376,10 @@ setTag obj t = obj{ tag = t } - + - + Restrictions @@ -1530,7 +1530,7 @@ declarations. Define your own instances! - + @@ -3372,7 +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 @@ -3492,7 +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. @@ -4087,15 +4086,39 @@ The class IsString is defined as: class IsString a where fromString :: String -> a -And the only predefined instance is the obvious one to make strings work as usual: +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 @@ -4226,6 +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. @@ -4845,7 +4876,7 @@ Because the preprocessor targets Haskell (rather than Core), - + Bang patterns <indexterm><primary>Bang patterns</primary></indexterm> @@ -4860,7 +4891,7 @@ than the material below. Bang patterns are enabled by the flag . - + Informal description of bang patterns @@ -4915,7 +4946,7 @@ is part of the syntax of let bindings. - + Syntax and semantics @@ -4989,7 +5020,7 @@ a module. - + Assertions <indexterm><primary>Assertions</primary></indexterm>