Simple Recursion in Haskell

Question

I am trying to understand recursion in Haskell. I want to write a simple recursive function with the following properties: if x > 50 then rec(x) = x - 5 and rec(x) = rec(rec(x+10)) otherwise. This is what I came up with:

rec x = if x > 50
    then x-5
    else rec (rec (x+10))

The not recursive part seems to work fine, but the recursion doesn't. Every number less than 50 just return 50. For example rec 60 = 55 rec 40 = 50 rec 25 = 50

Any suggestions on how to fix my code would be appreciated.

Irrelevant to your problem, but I'd look into guards. They'd make your function much cleaner (subjectively). — Carcigenicate
– Carcigenicate, Commented Sep 5, 2015 at 16:06
I would also advocate for guards (rec x | x > 50 = x - 5) and then I would suggest you also explain what you expect the function to return, because to me it seems working correctly. — jakubdaniel
– jakubdaniel, Commented Sep 5, 2015 at 18:34

gallais · Accepted Answer · 2015-09-05 15:59:17Z

3

For x <= 50, your function is equivalent to:

rec' x = if x `mod` 5 == 0
         then 50
         else 45 + x `mod` 5

This can be checked by running a simple test:

all (\ x -> rec x == rec' x) [0..50]

As a consequence, you need to feed an input which is not a multiple of 5 to get an answer distinct from 50.

answered Sep 5, 2015 at 15:59

gallais

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