调试Haskell

Question

I'm new to functional programming and I'd like to debug a recursive function to see why I am getting particular value as return value. How do I accomplish that? I found some answers on this site as well as on online, but I can't get my head wrap around the idea of doing that. Any help would be appreciated.

recur = \a -> if a>100 then  a-10 else recur (recur (a+11))

Answer 1

You could do this (using Debug.Trace ):

import Debug.Trace (trace)

recur a | trace ("recur " ++ show a) False = undefined
recur a = if a>100 then  a-10 else recur (recur (a+11))

This produces output each time the function recur is called (or rather, because Haskell is lazy, each time the result of applying recur is needed).

Sample output (in ghci):

*Main> recur 99
recur 99
recur 110
recur 100
recur 111
recur 101
91