I believe it should be something along the lines of: sumacc xs = summer xs 0 where summer [] n = n summer (x:xs) n = summer xs (x+n) I'm sure

I believe it should be something along the lines of: sumacc xs = summer xs 0 where summer [] n = n summer (x:xs) n = summer xs (x+n) I'm sure there are macros in some library somewhere that implement pattern matching in lisp (regardless of dialect).