[Haskell-cafe] Elementary question about Type Constraints
Daniil Elovkov
daniil.elovkov at googlemail.com
Mon Jul 9 10:03:49 EDT 2007
2007/7/9, lassoken <lassoken at gmail.com>:
> Hi,
>
> I'm trying to translate Example 2.3.3 (simple symbolic differentation) from
> Structure and Interpretation of Computer Programs into Haskell.
>
> Here is code that works (as far as see):
> ---
> data Term b = Var String | Const b | Sum (Term b) (Term b) | Prod (Term b)
> (Term b)
> ...
> --instance Show (Term b) where show = showTerm
> showTerm (Var x) = x
> showTerm (Const c) = show c
> showTerm (Sum a b) = "(" ++ showTerm a ++ "+" ++ showTerm b ++ ")"
> showTerm (Prod a b) = "(" ++ showTerm a ++ showTerm b ++ ")"
> ---
>
> Where should I put type constraint (Show b) to be able to define Term b as
> an instance of Show class?
>
> Actually, I would like to say that Term b is an instance of Show iff b is
> and not to put constraint on b.
>
instance Show b => Show (Term b) where
...
I believe it is in the Gentle introduction to Haskell.
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