Haskell syntax for instance

Question

I'm trying to make a pair an instance of Printable, but I can't figure out the correct syntax. My Printable is this:

class Printable a where
    toString :: a -> [Char]

instance Printable Bool where
    toString True = "true"
    toString False = "false"
instance Printable () where
    toString () = "unit type"

and my instance for pairs is this:

instance  Printable ( a, b ) where
    toString (a,b) = "(" ++ toString a ++ ","++ toString b ++ ")"

which, upon compiling, gives me a No instance for (Printable a) arising from a use of ‘toString’. What am I doing wrong?

BTW, this is a type error, not a syntax error.

Reid Barton
– Reid Barton

2015-10-12 13:22:19 +00:00
Commented Oct 12, 2015 at 13:22 — Reid Barton
– Reid Barton, Commented Oct 12, 2015 at 13:22

Lee · Accepted Answer · 2015-10-11 19:30:32Z

5

You need a and b to be instances of Printable:

instance (Printable a, Printable b) => Printable ( a, b ) where
    toString (a,b) = "(" ++ toString a ++ ","++ toString b ++ ")"

answered Oct 11, 2015 at 19:30

Lee

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1 Comment

Chiffa Over a year ago

That's it. Thanks. I should re-read some things about instances, probably.

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