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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