Data declaration with constraint

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Revision as of 23:07, 4 June 2013 by AndrewC (talk | contribs) (Added use of GADTs to add a context to data constructors.)
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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. I like to declare an instance for an type constructor class for the type constructor T but its methods require type constraints that depend on the particular type constructor T.


instance Vector T where
   add (Cons x) (Cons y) = Cons (x+y)    -- requires Num constraint on type a


In Haskell 98, 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.


You could use ghc's Generalised Algebraic Data Structures (GADTs) to add an implicit context to the data constructors.

There has been some discussion about whether it is sensible to want this.

A Haskell 98 workaround is to use multi-parameter type classes, where T a and a are separate arguments.

See also