May I suggest using FunctionalDependencies
https://wiki.haskell.org/Functional_dependencies
The class declaration is changed to
class Indexable idx a | idx -> a where
first :: idx -> a
This just means that *a* is fully determined by idx (your tuple).
Hence, instead of using as suggested
first $ Tuple3 (1::Int) 'a' False::Int
You can simplify and let the inference do its magic and use:
first $ Tuple3 1 'a' False
On Mon, Jan 25, 2016 at 8:42 AM, Imants Cekusins
Hello Daniel,
it works with these tweaks:
-- begin
{-# LANGUAGE MultiParamTypeClasses, FlexibleInstances, FlexibleContexts #-} module TupInst where
data Tuple3 a b c = Tuple3 a b c deriving (Show)
data Tuple2 a b = Tuple2 a b deriving (Show)
class Indexable idx a where first :: idx -> a
instance Indexable (Tuple2 a b) a where first (Tuple2 a0 b0) = a0
instance Indexable (Tuple3 a b c) a where first (Tuple3 a0 b0 c0) = a0
-- end
call it in ghci like this:
first $ Tuple3 (1::Int) 'a' False::Int _______________________________________________ Beginners mailing list Beginners@haskell.org http://mail.haskell.org/cgi-bin/mailman/listinfo/beginners