[Haskell-cafe] Proposal: Generic conditions for 'if' and 'case'

Wvv vitea3v at rambler.ru
Mon Sep 2 00:51:01 CEST 2013

I think it is an old idea, but nevertheless.
Now we have next functions:

if (a :: Bool) then x else y

case b of
a1 :: Bool -> x1
a2 :: Bool -> x2

Let we have generic conditions for 'if' and 'case':

class Boolean a where
    toBool :: a -> Bool

instance Boolean Bool where
   toBool = id

instance Boolean [a] where
   toBool [] = False
   toBool _ = True

instance Boolean (Maybe a) where
   toBool Nothing = False
   toBool _ = True

instance Boolean Int where
   toBool 0 = False
   toBool _ = True

if' (a :: Boolean b) then x else y

case' d of
a1 :: Boolean b1 -> x1
a2 :: Boolean b2 -> x2

It is very easy to implement to desugar:
if' a then ... == if toBool ( a ) then ...

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