Actually the problem lies in your definition of fz, it has the wrong type to be used in lam.
The Z you get out of fz as type Z b String, but you need it to have Z (a -> b) String so that when you strip off the Z you have a Y String (a -> b) matching the result type of lam.
To get there replace your definition of fz with:
> fz :: Z a String -> Z (a -> b) String
> fz = Z . Y . unY . f . unZ
In 6.10.2 I used {-# LANGUAGE FlexibleInstances, TypeSynonymInstances, MultiParamTypeClasses, ScopedTypeVariables #-}
and that compiled just fine.
On Thu, Jul 2, 2009 at 8:02 PM, Ahn, Ki Yung
<kyagrd@gmail.com> wrote:
Kim-Ee Yeoh wrote:
The add function illustrates the kind of do-sugaring we know and love
that I want to use for Symantics.
lam f = unZ $ do
show_c0 <- get
let
vname = "v" ++ show_c0
c0 = read show_c0 :: VarCount
c1 = succ c0
fz :: Z a String -> Z b String
fz = Z . f . unZ
put (show c1)
s <- (fz . return) vname
return $ "(\\" ++ vname ++ " -> " ++ s ++ ")"
Now with lam, I get this cryptic error message (under 6.8.2):
Occurs check: cannot construct the infinite type: b = a -> b
When trying to generalise the type inferred for `lam'
Signature type: forall a1 b1.
(Y String a1 -> Y String b1) -> Y String (a1 ->
b1)
Type to generalise: forall a1 b1.
(Y String a1 -> Y String b1) -> Y String (a1 ->
b1)
In the instance declaration for `Symantics (Y String)'
Both the two types in the error message are identical, which suggests
no generalization is needed. I'm puzzled why ghc sees an infinite type.
Any ideas on how to proceed?
kyagrd@kyavaio:~/tmp$ ghci EvalTaglessF.hs
GHCi, version 6.10.3: http://www.haskell.org/ghc/ :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer ... linking ... done.
Loading package base ... linking ... done.
[1 of 1] Compiling Main ( EvalTaglessF.hs, interpreted )
EvalTaglessF.hs:264:14:
Couldn't match expected type `b1' against inferred type `b'
`b1' is a rigid type variable bound by
the type signature for `fz' at EvalTaglessF.hs:263:31
`b' is a rigid type variable bound by
the type signature for `lam' at EvalTaglessF.hs:248:26
Expected type: Z b1 String
Inferred type: Z b String
In the expression: Z . f . unZ
In the definition of `fz': fz = Z . f . unZ
EvalTaglessF.hs:264:22:
Couldn't match expected type `a1' against inferred type `a'
`a1' is a rigid type variable bound by
the type signature for `fz' at EvalTaglessF.hs:263:17
`a' is a rigid type variable bound by
the type signature for `lam' at EvalTaglessF.hs:248:16
Expected type: Z a1 String
Inferred type: Z a String
In the second argument of `(.)', namely `unZ'
In the second argument of `(.)', namely `f . unZ'
Failed, modules loaded: none.
I hope this gives you a hint, if any. I am not exactly sure about how to solve this but I might try using scoped type variables extension somehow if I were in your shoe.