The error is because of the way records work in Haskell. Recall that a
record is just sugar for the normal datatype syntax. Namely:
data FooA a b c = FooA {getA :: a, getB:: b, getC :: c}
can be accessed as either
f (FooA a b c) = ...
or
f fooA = ... (getA fooA) ... etc
That is, Record syntax just creates functions for each label that take a
record and return the content of that label. eg
getA :: FooA a b c -> a
getA (FooA a _ _ ) = a
...
So when you have two records with the same label in it:
data Bar = Bar { badlabel :: Int }
data Baz = Baz { badlabel :: String }
even though they are not the same type, you end up with the following
definitions:
badlabel :: Bar -> Int
badlabel :: Baz -> String
this is a type error, one that is not trivially resolved. Thats where
your problem is coming from, two fields both named `x` which result in
this error.
HTH,
/Joe
John Ky wrote:
Hi Luke,
You're right. My code had a typo. Unfortunately, I still get the
same error whichever way I do it.
For example:
> {-# LANGUAGE DisambiguateRecordFields #-}
> import A
> import B
>
> main = do
> let xx = getA
> print (x xx)
and:
#!/usr/bin/env runhaskell
> {-# LANGUAGE DisambiguateRecordFields #-}
> import A
> import B
>
> main = do
> let xx = getA
> putStrLn $ show (x xx)
both give me:
test.lhs:8:22:
Ambiguous occurrence `x'
It could refer to either `A.x', imported from A at test.lhs:3:2-9
(defined at A.hs:5:5)
or `B.x', imported from B at test.lhs:4:2-9
(defined at B.hs:5:5)
Any ideas?
$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.10.3
Thanks,
-John
On Sat, Jun 6, 2009 at 6:41 PM, Luke Palmer <lrpal...@gmail.com
<mailto:lrpal...@gmail.com>> wrote:
On Sat, Jun 6, 2009 at 1:48 AM, John Ky <newho...@gmail.com
<mailto:newho...@gmail.com>> wrote:
Hi Haskell Cafe,
In the following code, I get an error saying Ambiguous
occurrence `x'. Why can't Haskell work out which x to call
based on the type of getA?
Thanks
-John
#!/usr/bin/env runhaskell
> {-# LANGUAGE DisambiguateRecordFields #-}
> import A
> import B
>
> main = do
> let xx = getA
> putStrLn $ show x xx
This is parsed as two arguments passed to the show function (which
only takes one argument).
putStrLn $ show (x xx)
Or because putStrLn . show = print;
print $ x xx
----------------------
module A where
data TypeA = TypeA
{ a :: Int
, x :: Int
}
getA = TypeA { a = 1, x = 2 }
-------------------------
module B where
data TypeB = TypeB
{ b :: Int
, x :: Int
}
getB = TypeB { b = 1, x = 3 }
--------------------------
./test.lhs:8:21:
Ambiguous occurrence `x'
It could refer to either `A.x', imported from A at
./test.lhs:3:2-9
(defined at A.hs:5:5)
or `B.x', imported from B at
./test.lhs:4:2-9
(defined at B.hs:5:5)
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