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 Oct 12 '15 at 13:22 — Reid Barton, Oct 12 '15 at 13:22

score 5 · Accepted Answer · answered Oct 11 '15 at 19:30

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 '15 at 19:30

Lee

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That's it. Thanks. I should re-read some things about instances, probably. – Chiffa Oct 11 '15 at 19:38

Haskell syntax for instance

1 Answers1