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Today's Topics:
1. Problems with Data types a la carte (Rodrigo Geraldo)
2. Re: Problems with Data types a la carte (Daniel Fischer)
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Message: 1
Date: Fri, 23 Apr 2010 17:52:19 -0300
From: Rodrigo Geraldo <[email protected]>
Subject: [Haskell-beginners] Problems with Data types a la carte
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Message: 2
Date: Fri, 23 Apr 2010 23:35:54 +0200
From: Daniel Fischer <[email protected]>
Subject: Re: [Haskell-beginners] Problems with Data types a la carte
To: [email protected]
Message-ID: <[email protected]>
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Am Freitag 23 April 2010 22:52:19 schrieb Rodrigo Geraldo:
> Hi !
>
> I've tried to implement the expression interpreter present in Data types
> a la carte, putting the new "cases" of my data type in a separate
> module. When I'm trying to create a simple
> expression like this:
>
> x :: Expr (Val :+: Add :+: Mult)
> x = (val 2000 `mult` val 399) `plus` (val 3)
>
> I'm getting the following type error that I can't solve:
>
Your problem is the wrong associativity, the type signature is parsed as
Expr ((Val :+: Add) :+: Mult)
while you need
Expr (Val :+: (Add :+: Mult))
Insert the parentheses or add a fixity declaration
infixr k :+:
to Expr.hs (where k must be replaced with an integer from 0 through 9).
>
> Regards
>
> Rodrigo
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