X-Git-Url: http://git.megacz.com/?a=blobdiff_plain;f=docs%2Fusers_guide%2Fglasgow_exts.xml;h=d9a6198788717b1d2299784935c4a47e0e0dfb50;hb=78b620de24ceb4621cead67bd642f72749b41052;hp=a8c91dff8f31d55f6e5281e7f016a108ce0b49fc;hpb=041c35e5d9f3aefab6908b4c776a3fd8242720ab;p=ghc-hetmet.git diff --git a/docs/users_guide/glasgow_exts.xml b/docs/users_guide/glasgow_exts.xml index a8c91df..d9a6198 100644 --- a/docs/users_guide/glasgow_exts.xml +++ b/docs/users_guide/glasgow_exts.xml @@ -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 @@ -2546,7 +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 @@ -2554,7 +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 @@ -2566,11 +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 @@ -2622,11 +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. - @@ -2691,8 +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.