Data declaration with constraint
(maybe multi-parameter type classes are a way out)
(Added use of GADTs to add a context to data constructors.)
Latest revision as of 23:07, 4 June 2013
 1 Problem
 1.1 Question
I have declared
data C a => T a = Cons a
and I hoped that now the type checker knows,that every value of type
instance Vector T where add (Cons x) (Cons y) = Cons (x+y) -- requires Num constraint on type a
 1.2 Answer
In Haskell 98, only functions can have type constraints.The type constraint of a
The designers of Haskell 98 do now think, that it was a bad decision to allow constraints on constructors.
 2 Solution
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