How to use my recursive function in Haskell?

Question

lol :: IO int
lol = go -1 0 0 0
where go k m n p = case k of
        -1 -> do{
                q <- readLn ;
                go 0 0 0 q
            }
        p -> do{
                putStrLn $ show $ n
            }
        _ -> do{
                q <- readLn ;
                go (k+1) q (if q > m then n+q-m else n) p
                return()
            }

I(a haskell beginner) am doing some practise questions. I have written a recursive function but i don't know how to use it in main. Please help me to do solve this problem. This is my idea in C++ and I got it accepted.

#include<iostream>
using namespace std;
int main(){
    long long int prev=0;
    long long int count,ans=0;
    cin >> count;
    long long int p;
    for(int i=0; i<count ;i++){
        cin >> p;
        if(p>prev){
            ans+=p-prev;
        }
        prev=p;
    }
    cout << ans << endl;
}

What's the purpose of the function? Explain. In Haskell, recursive functions usually require at least two parameters where there's the head, and the tail of a set of elements. — Poriferous
– Poriferous, Commented Feb 12, 2016 at 3:07

Daniel Wagner · Accepted Answer · 2016-02-12 03:23:14Z

2

There are several syntax errors:

add parentheses around negative numbers when using them as arguments
to avoid starting a fresh line in the module block, indent where (and all following lines) a little bit
add the missing semicolon to the line before return ()

And one type error: you should write lol :: IO (). After fixing these, you may write main = lol to use it (or simply name it main instead of lol). No idea whether it does what it's supposed to do, but that seems like a thing you should be able to test once you get it running.

answered Feb 12, 2016 at 3:23

Daniel Wagner

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Comments

user2407038 · Accepted Answer · 2016-02-12 04:01:12Z

2

How about this. I just translated it directly from C++

module Main where


go :: Int -> Int -> Int -> IO Int
go i prev acc
  | i > 0   = do p <- readLn
                 if p > prev
                   then go (i-1) p (acc + (p - prev))
                   else go (i-1) p acc
  | otherwise = return acc

main :: IO ()
main = do
  count <- readLn :: IO Int
  ans <- go count 0 0
  print ans

edited Feb 12, 2016 at 4:01

user2407038

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answered Feb 12, 2016 at 3:25

lehins

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4 Comments

dfeuer Over a year ago

The way you use fmap there will pile up a bunch of "stack". You should make ans an argument to go (and make it strict). Or use StateT or (less efficient) an IORef.

lehins Over a year ago

@dfeuer, since you ask :) I did not make acc strict, after all, we are striking for clarity hear, not performance.

dfeuer Over a year ago

With BangPatterns, that's as simple as go i prev !acc = .... But I guess you don't feel like a language pragma.

lehins Over a year ago

@dfeuer Precisely. :)

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