[Haskell] Signature error because the order
simonpj at microsoft.com
Thu Jan 14 11:58:32 EST 2010
In Haskell, the context of all functions in a mutually recursive group must be identical.
From: haskell-bounces at haskell.org [mailto:haskell-bounces at haskell.org] On Behalf Of Luis Cabellos
Sent: 14 January 2010 11:26
To: haskell at haskell.org
Subject: [Haskell] Signature error because the order
I found than this piece of code causes an error in GHC ( http://haskell.pastebin.com/m5e3f4a4c ):
data Tree a = Leaf a | Node (Tree a) (Tree a)
--prettyShow :: (Show a, Num a) => Tree a -> String -- WORKS
prettyShow :: (Num a, Show a) => Tree a -> String -- FAILS
prettyShow (Leaf a) = show a
prettyShow (Node a b) = (simple a) ++ (simple b)
simple :: (Show a, Num a) => Tree a -> String
simple (Leaf x) = prettyShow (Leaf x)
simple x = "(" ++ prettyShow x ++ ")"
With the error:
Couldn't match expected type `Show a'
against inferred type `Num a1'
When matching the contexts of the signatures for
prettyShow :: forall a. (Num a, Show a) => Tree a -> String
simple :: forall a. (Show a, Num a) => Tree a -> String
The signature contexts in a mutually recursive group should all be identical
When generalising the type(s) for prettyShow, simple
Failed, modules loaded: none.
But if I change the order of Show and Num in the signature of prettyShow it works.
It's a GHC problem? it's normal than signature is order-dependent?
I use GHC 6.10.3
Thanks, Luis Cabellos
LC, ("There is no Dana, only Zuul." - Cazafantasmas )
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