Bugs item #1146068, was opened at 2005-02-22 10:11
Message generated for change (Comment added) made by p1738j
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Category: Compiler (Type checker)
Group: 6.4
Status: Open
Resolution: None
Priority: 5
Submitted By: Nobody/Anonymous (nobody)
Assigned to: Nobody/Anonymous (nobody)
Summary: Data.Generics type error
Initial Comment:
I get a strange type error when using
Data.Generics.Twins - but if I include the text of the
definition in my own file it works!
I attach two files, one type checks, one does not. The
only difference is that I move a definition between
modules.
/Patrik
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Comment By: Patrik Jansson (p1738j)
Date: 2005-02-22 10:18
Message:
Logged In: YES
user_id=1151788
The error is the following:
Compiling Main ( testeq_bad.hs, interpreted )
testeq_bad.hs:21:34:
Inferred type is less polymorphic than expected
Quantified type variable `a' escapes
Expected type: a1 -> GenericQ Bool
Inferred type: a1 -> a -> Bool
In the first argument of `gzipWithQ', namely `geq''
In the first argument of `and', namely `(gzipWithQ geq'
x y)'
Failed, modules loaded: none.
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Comment By: Patrik Jansson (p1738j)
Date: 2005-02-22 10:16
Message:
Logged In: YES
user_id=1151788
-- I did not manage to later add the second file - here it
is instead:
-- I tested this with ghci-6.4.20050214
-----
{-# OPTIONS -fglasgow-exts #-}
import Data.Generics.Basics
import Data.Generics.Aliases
import Data.Generics.Twins(gmapAccumQ)
-- | Twin map for queries
gzipWithQ :: GenericQ (GenericQ r) -> GenericQ (GenericQ [r])
gzipWithQ f x y = case gmapAccumQ perkid funs y of
([], r) -> r
_ -> error "gzipWithQ"
where
perkid a d = (tail a, unGQ (head a) d)
funs = gmapQ (\k -> GQ (f k)) x
-- | Generic equality: an alternative to \deriving Eq\
geq :: Data a => a -> a -> Bool
geq x y = geq' x y
where
geq' :: forall a b. (Data a, Data b) => a -> b -> Bool
geq' x y = (toConstr x == toConstr y)
&& and (gzipWithQ geq' x y)
----------------------------------------------------------------------
Comment By: Patrik Jansson (p1738j)
Date: 2005-02-22 10:13
Message:
Logged In: YES
user_id=1151788
This was submitted by me (Patrik Jansson = p1738j at sf.net)
- I just loggon on too late.
----------------------------------------------------------------------
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