On Mon, Mar 26, 2012 at 4:25 PM, Ting Lei <[email protected]> wrote: > Hi Antoine and Tobias (and everyone else), > > Thanks a lot for your answers. They are really helpful > > Can you please show me how to use the (Eq m) constraint to do this? > > Also, my general question (probably novice-level) is that in monadic > programming, you can convert not necessarily monadic codes into monadic > ones. > I know for many cases, it is impossible to do the reverse conversion, e.g. > you can't make a function involving real IO operations into a pure code. > In other cases, for example, I may need to using things like Nothing as the > "null" value as in other programming languages, just to represent a special > "missing" value outside the regular type. > Is mzero a reasonable replacement for this or is there any reasonable > (abstract) approximation in Haskell for doing this? (Like "null", I need the > ability to detect it.)
I think using 'Maybe' (with Nothing) is perfect for this - this function should come in handy: http://hackage.haskell.org/packages/archive/base/latest/doc/html/Data-Maybe.html#v:isNothing Antoine _______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
