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Today's Topics:
1. Problem with typeclass (Sebastien Geffroy)
2. Re: Problem with typeclass (Stephen Tetley)
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Message: 1
Date: Sun, 20 Jun 2010 10:32:46 +0200
From: Sebastien Geffroy <s...@geffroy.info>
Subject: [Haskell-beginners] Problem with typeclass
To: beginners@haskell.org
Message-ID: <13d082e6-6431-470d-98dd-423f2e9c6...@geffroy.info>
Content-Type: text/plain; charset=us-ascii
Hi all,
Below is an example from the book "Real world haskell"
data JValue = JString String
| JBool Bool
| JNumber Int
deriving(Show)
class JSON a where
toJValue :: a -> JValue
fromJValue :: JValue -> Either String a
instance JSON JValue where
toJValue = id
fromJValue = Right
instance JSON Bool where
toJValue = JBool
fromJValue (JBool a) = Right a
fromJValue _ = Left "Not a JValue"
I'm trying to test this in ghci :
> fromJValue (JBool True)
and i got
<interactive>:1:0:
Ambiguous type variable `a' in the constraint:
`JSON a' arising from a use of `fromJValue' at <interactive>:1:0-22
Probable fix: add a type signature that fixes these type variable(s)
I don't understand where i have to declare a type signature ? in the class
definition for "JSON a" ? in the instance declaration for "JSON Bool" ?
Thanks in advance.
Sheb
------------------------------
Message: 2
Date: Sun, 20 Jun 2010 11:03:02 +0100
From: Stephen Tetley <stephen.tet...@gmail.com>
Subject: Re: [Haskell-beginners] Problem with typeclass
To: Sebastien Geffroy <s...@geffroy.info>
Cc: beginners@haskell.org
Message-ID:
<aanlktimnva_dnzyoey8ofyzwjxja_jdfsrmcol6p8...@mail.gmail.com>
Content-Type: text/plain; charset=ISO-8859-1
Hello Sebastien
Try
> (fromJValue (JBool True)) :: Bool
Or
> (fromJValue (JBool True)) :: JValue
The class function /fromJValue/ is polymorphic on its answer, so you
need to tell GHC / GHCi what type the answer will be. GHC can't infer
it because it could be more than one type.
Best wishes
Stephen
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