Haskell parse error on input `return'

Question

I want to get a random prime from a prime array name "primes". So I want to generate a random integer first, and then get the prime at that Integer position.

Here is my code:

getPrime :: IO Integer
getPrime =  do 
        pos <- getStdRandom (randomR (10, 100))
        return (primes !! pos)

(primes already exists and works well.)

But when I load this code, it says "parse error on input `return'"

Can anyone help me about this? Thx!!!!!!!!!!!!!!!

After replacing all the undefined names with `undefined` your code loads without error for me (GHC 7.6.3). So, the problem may be with the code that you did not paste. — ntc2, Oct 12 '13 at 09:02

score 3 · Accepted Answer · answered Oct 12 '13 at 11:58

3

The problem must be with whitespace. You must have mixed tabs with spaces in the indentation. To protect yourself from that kind of situation it's conventional in Haskell to only indent with spaces.

answered Oct 12 '13 at 11:58

Nikita Volkov

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haha~ thank u! i delete all the whitespaces and replace them with just one space. and it works!! – zll19910516 Oct 12 '13 at 13:12

score 2 · Answer 2 · answered Oct 12 '13 at 09:12

The problem isn't in the given code as the following code works without trouble:

import System.Random

primes = [1..100]

getPrime :: IO Integer
getPrime = do
      pos <- getStdRandom (randomR (10, 100))
      return (primes !! pos)

main = getPrime

I would suggest starting with this and extending it piece by piece to see where the problem is.

Haskell parse error on input `return'

2 Answers2