[This bug report may be related to SimonPJ's question about implicit
parameters and the monomorphism restriction - I'm not sure.]
The attached program contains two definitions of "ident" - a function
that uses implicit parameters.
The only difference between them is that the first (which works) binds
the implicit parameter to a normal variable and the second (which
gives a type error uses the implicit parameter directly).
$ rm -f T.o && ghc -c -fglasgow-exts T.hs -cpp -DOK=1
$ rm -f T.o && ghc -c -fglasgow-exts T.hs -cpp -DOK=2
T.hs:20:
Could not deduce `?env :: ([(String, b)], b1)' from the context ()
Probable fix:
Add `?env :: ([(String, b)], b1)' to the banding(s) for {y}
Or add an instance declaration for `?env :: ([(String, b)], b1)'
arising from use of implicit parameter `?env' at T.hs:20
In the first argument of `fst', namely `env'
In the second argument of `lookup', namely `(fst env)'
Interestingly, both versions work if I remove the 2nd component of Env:
type Env = ([(String,Int)])
and delete the calls to fst. This makes the somewhat mysterious
behaviour of the typechecker even more mysterious.
--
Alastair Reid [EMAIL PROTECTED] http://www.cs.utah.edu/~reid/
module T where
import Maybe
type Env = ([(String,Int)],Int)
ident :: (?env :: Env) => String -> Int
#if OK==1
ident x = y
where
env = ?env
y = fromJust (lookup x (fst env))
#elif OK==2
ident x = y
where
y = fromJust (lookup x (fst ?env))
#endif
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