Data declaration with constraint

From HaskellWiki
Revision as of 17:36, 21 December 2007 by Lemming (talk | contribs) (constraint must be in front of the type)
Jump to: navigation, search



I have declared

data C a  =>  T a = Cons a

and I hoped that now the type checker knows, that every value of type T a satisfies the type constraint on a.


Only functions can have type constraints. The type constraint of a data only refers to the constructors. The designers of Haskell 98 do now think, that it was a bad decision to allow constraints on constructors.


But how can one bake type constraints into a type ? ...

See also