Neil Mitchell wrote:
Hi,
First, its useful to define referential transparency.
In Haskell, if you have a definition
f = not
Then this means that anywhere you see f, you can replace it with not. For example
"f True" and "not True" are the same, this is referentially transparent.
Now lets define "super show" which takes a function, and prints its code behind it, so:
superShow f = "not" superShow g = "\x -> case ..."
now superShow f /= superShow g, so they are no longer referentially transparent.
Hence, you have now broken referential transparency.
So you can't show these two functions differently, so what can you do instead? You can just give up and show all functions the same
instance Show (a -> b) where show x = "<function>"
This is the constant definition of show mentioned.
You can be less restrictive than that. The show function can give different results when applied to functions with different types without breaking anything. -- Lennart