[Haskell-cafe] "show" for functional types

Brian Hulley brianh at metamilk.com
Sat Apr 1 11:53:53 EST 2006

Claus Reinke wrote:
> the usual way to achieve this uses the overloading of Nums in Haskell:
> when you write '1' or '1+2', the meaning of those expressions depends
> on their types. in particular, the example above uses 'T Double', not
> just 'Double'.

However there is nothing in the functions themselves that restricts their 
use to just T Double. Thus the functions can be compared for equality by 
supplying an argument of type T Double but used elsewhere in the program 
with args of type (plain) Double eg:

-- Change to module AbsNum
instance (Simplify a)=>Eq (T a) where
    (==) (Const x) (Const y) = x == y
    (==) (Var x) (Var y) = x == y
    (==) (Add xs) (Add ys) = and (map (\(x, y) -> x==y) (zip xs ys))
    (==) _ _ = False -- Not needed for the example

module Main where
import AbsNum

f x = x + 2.0
g x = x + 1.0 + 1.0

funeq :: (T Double -> T Double) -> (T Double -> T Double) -> Bool
funeq p q = let y = Var "y" in p y == q y

main = do
 print (funeq f g)
 print (f 10)
 print (g 10)
 putStrLn ""
 print (funeq f f)
 print (f 10)
 print (g 10)



Thus we can determine that f is implemented by different code from g (The 
example would be even more convincing if Int's were used instead of 
Double's) and so f and g are not interchangeable.

> ... nothing prevents us from defining that instance in such a way that we 
> construct a
> representaton instead of doing any additions.

Thus referential transparency of polymorphic functions is foiled.

Regards, Brian. 

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