haskell - Strange pattern matching with functions instancing Show -

so i'm writing program returns procedure given arithmetic problem, wanted instance couple of functions show can print same expression evaluate when test. trouble given code matches (-) first line when should fall second.

{-# options_ghc -xflexibleinstances #-}  instance show (t -> t-> t)    show (+) = "plus"  show (-) = "minus"    main = print [(+),(-)] 

returns

[plus,plus] 

am committing mortal sin printing functions in first place or there way can match properly?

edit:i realise getting following warning:

warning: pattern match(es) overlapped          in definition of `show': show - = ... 

i still don't know why overlaps, or how stop it.

as sepp2k , mtnviewmark said, can't pattern match on value of identifiers, on constructors and, in cases, implicit equality checks. so, instance binding argument identifier, in process shadowing external definition of (+). unfortunately, means you're trying won't , can't ever work.

a typical solution want accomplish define "arithmetic expression" algebraic data type, appropriate show instance. note can make expression type instance of num, numeric literals wrapped in "literal" constructor, , operations (+) returning arguments combined constructor operation. here's quick, incomplete example:

data expression = literal                   | sum (expression a) (expression a)                   | product (expression a) (expression a)                   deriving (eq, ord, show)  instance (num a) => num (expression a)     x + y = sum x y     x * y = product x y     frominteger x = literal (frominteger x)  evaluate (literal x) = x evaluate (sum x y) = evaluate x + evaluate y evaluate (product x y) = evaluate x * evaluate y  integer :: integer integer = (1 + 2) * 3 + 4  expr :: expression integer expr = (1 + 2) * 3 + 4 

trying out in ghci:

> integer 13 > evaluate expr 13 > expr sum (product (sum (literal 1) (literal 2)) (literal 3)) (literal 4)