has anybody here used in a non-trivial way the showsPrec anti-parser?
Isn't the idea to make things trivial while avoiding performance penalties? Perhaps: simple pretty-printing of abstract syntax trees? I often use it to get simple debugging output for complex internal data structures (first, use deriving; then, define showsPrec; if that's still not good enough, do some real thinking..). Anyway, this reminded me of a litte old hack of mine. Only trivial use of showsPrec, but perhaps you'll like it anyway?-) http://www.cs.ukc.ac.uk/people/staff/cr3/toolbox/haskell/R.hs As with anything else in my toolbox, no warranty for nothing.. Cheers, Claus ------------------- cut here {- Representative thingies.. A little hack to pair values with string representations of their expressions. Useful if you want to explain what map (+1) [1..4] or foldr1 (*) [1..5] do, or if you want to demonstrate the difference between foldr (+) 0 [1..4] and foldl (+) 0 [1..4] Load this module into Hugs (Hugs mode) and type in some of these examples to get an idea of what I mean. Also try map (+) [1..4] This could be extended in various directions, but I wanted to keep things simple. I'm not convinced that extra complications would be worth the effort. Claus Reinke -} default (R Integer) data R a = R {rep:: String ,val:: a } instance Show (R a) where showsPrec _ a = showString (rep a) instance Show (R a -> R b) where showsPrec _ f = showString ("\\x->"++(rep (f x))) where x = R{rep="x",val=error "variable"} instance Show (R a -> R b -> R c) where showsPrec _ f = showString ("\\x y->"++(rep (f x y))) where x = R{rep="x",val=error "variable"} y = R{rep="y",val=error "variable"} lift1 op a = R {rep="("++(rep op)++" "++(rep a)++")" ,val= ( (val op) (val a) ) } lift2 op a b = R {rep="("++(rep op)++" "++(rep a)++" "++(rep b)++")" ,val= ( (val op) (val a) (val b) ) } lift2infix op a b = R {rep="("++(rep a)++" "++(rep op)++" "++(rep b)++")" ,val= ( (val a) `iop` (val b) ) } where iop = val op instance (Num a,Show a) => Num (R a) where (+) = lift2infix R{rep="+",val=(+)} (-) = lift2infix R{rep="-",val=(-)} (*) = lift2infix R{rep="*",val=(*)} negate = lift1 R{rep="-",val=negate} fromInteger a = (\fIa->R{rep=show fIa,val=fIa}) (fromInteger a) instance (Eq a,Num a) => Eq (R a) where a == b = (val a) == (val b) instance (Ord a,Num a) => Ord (R a) where a <= b = (val a) <= (val b) instance (Enum a,Num a,Show a) => Enum (R a) where fromEnum = fromEnum.val toEnum a = R{rep=show a,val=toEnum a} enumFrom x = map toEnum [fromEnum x..] -- missing in Hugs Prelude..