Difference between revisions of "Data declaration with constraint"
From HaskellWiki
(short introduction with reference to current discussion) |
(constraint must be in front of the type) |
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| Line 5: | Line 5: | ||
I have declared | I have declared | ||
<haskell> | <haskell> | ||
| − | data | + | data C a => T a = Cons a |
</haskell> | </haskell> | ||
and I hoped that now the type checker knows, that every value of type <hask>T a</hask> satisfies the type constraint on <hask>a</hask>. | and I hoped that now the type checker knows, that every value of type <hask>T a</hask> satisfies the type constraint on <hask>a</hask>. | ||
Revision as of 17:36, 21 December 2007
Contents
Problem
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 T a satisfies the type constraint on a.
Answer
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.
Solution
But how can one bake type constraints into a type ? ...
See also
- Henning Thielemann in Haskell-Cafe: Context for type parameters of type constructors
- Mark Nicholls in Haskell-Cafe: nice simple problem for someone struggling....