On Mon, Oct 6, 2008 at 2:19 PM, Arnar Birgisson
And this requirement is there why? Is it specifically put in so that one is able to create this overhead-less implementation?
Given:
data A = A Int newtype B = B Int
ta (A x) = True tb (B x) = True
This happens (not surprisingly given your above comments):
*Main GOA> :load test.hs [1 of 1] Compiling Main ( test.hs, interpreted ) Ok, modules loaded: Main. *Main GOA> ta undefined *** Exception: Prelude.undefined *Main GOA> tb undefined True
Why is the x evaluated in ta?
x isn't evaluated. "undefined" is evaluated to see if it matches the constructor "A". But we don't even get to check, because undefined throws an exception during its evaluation. In the "tb" case, (B x) always matches because B is a newtype. x gets bound to undefined, but never evaluated. -- ryan