X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=docs%2Fusers_guide%2Fglasgow_exts.xml;h=36bb71ce71a03185c8cfb1a3239e899a1ed5e0d8;hb=1f28aaa6116d9cb90966b1bb6cdcbe52fe938867;hp=dfedb083b537dd66f0e0e6d8736f6201f4a0f74b;hpb=a8e681c1e8aa4bc602714ff61583cd4e969d7187;p=ghc-hetmet.git diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index dfedb08..36bb71c 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -552,14 +552,11 @@ 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. @@ -1330,7 +1327,7 @@ 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 @@ -2546,7 +2543,7 @@ 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 @@ -2554,7 +2551,7 @@ 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 @@ -2566,11 +2563,15 @@ 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 @@ -2622,11 +2623,6 @@ 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. - @@ -2691,8 +2687,8 @@ 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. @@ -4086,15 +4082,39 @@ 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