Hi, On Mon, Oct 6, 2008 at 16:10, Ryan Ingram <[EMAIL PROTECTED]> wrote: > On Mon, Oct 6, 2008 at 2:19 PM, Arnar Birgisson <[EMAIL PROTECTED]> wrote: >> 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.
Yes, realized my error just after hitting send :/ > "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. And this happens because data values are basically pattern matched at run-time but newtype values are matched at compile-time, effectively turning tb into an Int -> Bool function? That explains pretty well why newtype can have only one constructor. cheers, Arnar _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
