haskell call function of certain instance

Question

I have a class :

> class Binaer a where
>   binaer_zu_a :: String -> a

And two instances:

> instance Binaer Integer where
>   binaer_zu_a b = 3

> instance Binaer Bool where
>   binaer_zu_a b = True

So the input to he function binaer_zu_a is always a string, but how do I have to call the function so that the function "knows" which of them it should execute?

If I try to call it like binaer_zu_a :: Bool "01" for example I ge anERROR - Syntax error in input (unexpected string literal)

So how do I need to call that function?

If you annotate the function, you need to do so with a function type: (binaer_zu_a :: String -> Bool) "True". — chepner
– chepner, Commented Nov 3, 2019 at 16:41

Brady Dean · Accepted Answer · 2019-11-03 16:25:41Z

9

Specify the type argument like this

> (binaer_zu_a "b") :: Integer
3
> (binaer_zu_a "b") :: Bool
True

answered Nov 3, 2019 at 16:25

Brady Dean

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chepner · Accepted Answer · 2019-11-03 16:42:58Z

3

With the TypeApplications extension, you can specify the type (or rather, an expression representing the type) as the first argument.

> :set -XTypeApplications
> binaer_zu_a @Bool "True"
True

answered Nov 3, 2019 at 16:42

chepner

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