Haskell type classes and type families (cont'd)
- by Giuseppe Maggiore
I need some help in figuring a compiler error which is really driving me nuts...
I have the following type class:
infixl 7 -->
class Selectable a s b where
type Res a s b :: *
(-->) :: (CNum n) => (Reference s a) -> (n,(a->b),(a->b->a)) -> Res a s b
which I instance twice. First time goes like a charm:
instance Selectable a s b where
type Res a s b = Reference s b
(-->) (Reference get set) (_,read,write) =
(Reference (\s ->
let (v,s') = get s
in (read v,s'))
(\s -> \x ->
let (v,s') = get s
v' = write v x
(_,s'') = set s' v'
in (x,s'')))
since the type checker infers
(-->) :: Reference s a -> (n,a->b,a->b->a) -> Reference s b
and this signature matches with the class signature for (--) since
Res a s b = Reference s b
Now I add a second instance and everything breaks:
instance (Recursive a, Rec a ~ reca) => Selectable a s (Method reca b c) where
type Res a s (Method reca b c) = b -> Reference s c
(-->) (Reference get set) (_,read,write) =
\(x :: b) ->
from_constant( Constant(\(s :: s)->
let (v,s') = get s :: (a,s)
m = read v
ry = m x :: Reference (reca) c
(y,v') = getter ry (cons v) :: (c,reca)
v'' = elim v'
(_,s'') = set s' v''
in (y,s''))) :: Reference s c
the compiler complains that
Couldn't match expected type `Res a s (Method reca b c)'
against inferred type `b -> Reference s c'
The lambda expression `\ (x :: b) -> ...' has one argument,
which does not match its type
In the expression:
\ (x :: b)
-> from_constant (Constant (\ (s :: s) -> let ... in ...)) ::
Reference s c
In the definition of `-->':
--> (Reference get set) (_, read, write)
= \ (x :: b)
-> from_constant (Constant (\ (s :: s) -> ...)) :: Reference s c
reading carefully the compiler is telling me that it has inferred the type of (--) thusly:
(-->) :: Reference s a -> (n,a->(Method reca b c),a->(Method reca b c)->a) -> (b -> Reference s c)
which is correct since
Res a s (Method reca b c) = b -> Reference s c
but why can't it match the two definitions?
Sorry for not offering a more succint and standalone example, but in this case I cannot figure how to do it...
