I am currently working through SICP with Haskell. Exercise 1.15 asks how many times a function is called. The idea is probably that you should use the substitution method, but I would like to know how to do so in code.
In an imperative language one can keep a global variable and increment it every time the function is called. But how would you go about it in Haskell (or the pure functional way)?
You can use the Writer monad to accomplish this, provided that all of the calls to the function in question can be grouped together into a do block:
import Control.Monad.Writer
myFunc :: Int -> Int -> Writer (Sum Int) Int
myFunc a b = tell (Sum 1) >> return (a + b)
callMyFunc :: ((Int, Int, Int), Sum Int)
callMyFunc = runWriter $ do a <- myFunc 2 3
b <- myFunc 8 7
c <- myFunc 3 5
return (a, b, c)
main = putStrLn $
"myFunc was called "
++ show (getSum $ snd callMyFunc)
++ " times and produced "
++ show (fst callMyFunc)
Which outputs:
myFunc was called 3 times and produced (5,15,8)