Data declaration with constraint

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Current revision

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

E.g.

instance Vector T where
   add (Cons x) (Cons y) = Cons (x+y)    -- requires Num constraint on type 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? You cannot. One should insert a discussion here, whether it is sensible to want this.

For now you have to stick to multi-parameter type classes,

where

T a

and

a

are separate arguments.

@@ Line 7: / Line 7: @@
 data C a  =>  T a = Cons a
 </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>.
+I like to declare an instance for an type constructor class for the type constructor <hask>T</hask>
+but its methods require type constraints that depend on the particular type constructor <hask>T</hask>.
+E.g.
+<haskell>
+instance Vector T where
+   add (Cons x) (Cons y) = Cons (x+y)    -- requires Num constraint on type a
+</haskell>
 === Answer ===
@@ Line 17: / Line 26: @@
 == Solution ==
-But how can one bake type constraints into a type ? ...
+But how can one bake type constraints into a type?
+You cannot. One should insert a discussion here, whether it is sensible to want this.
+For now you have to stick to [[multi-parameter type class]]es,
+where <hask>T a</hask> and <hask>a</hask> are separate arguments.