Is it possible to define a function on a subset of an existing type?

Question

I\'m new to Haskell and would like to know whether it\'s possible to define a function that is only defined on a subset of an already existing type, without actually having

Answer 1

As mentioned elsewhere, refinement types like in LiquidHaskell can express this. Here's what it looks like:

module Evens where

{-@ type Even = {v:Int | v mod 2 = 0} @-}

{-@ square :: Even -> Int @-}
square :: Int -> Int
square n = n * n

-- calling the function:
yup = square 4
-- nope = square 3 -- will not compile if this is uncommented

You can try this out by plugging it in here: http://goto.ucsd.edu:8090/index.html