Parse Error on input 'sum' in haskell

Question

I'm writing a short program in haskell to find the mean of the list of odd numbers between 1 and 2000 that are not divisible by 3 or 5. I can't get it to compile and keep getting a variety of errors. I made some changes and now the code is giving me a "Parse error on input 'sum'" on line 5 col 9. Could someone tell me what I'm doing wrong please?

--Write a Haskell function that calculates the mean of a list of odd numbers that
--are not divisible by 3 or 5 and whose sum is less than 2000.
mean :: Int
mean = let nums = [x|x <- [1,3..1999], x 'mod' 3 != 0, x 'mod' 5 != 0]
    sum nums/length nums

I'm compiling with GHCI. Thanks

Is this homework, and is the comment in the code the original task description? If so, I'd say you have misunderstood it. — chirlu
– chirlu, Commented Aug 7, 2013 at 19:39
Yes the comment at the top is the original question. It's from a past exam paper that I was working through to study for an exam. Why do you think it's wrong? — user1786327
– user1786327, Commented Aug 8, 2013 at 10:32

Jan Christiansen · Accepted Answer · 2013-08-09 20:11:38Z

4

Besides the missing in as mentioned by DiegoNolan there are some other minor problems with your definition. First, to use a two-ary function infix, you have to enclose it in backticks ` while you use ticks '. Furthermore, Haskell uses /= for inequality and not !=. Finally, you cannot use / for integer division but function div.

mean = let nums = [x|x <- [1,3..1999], x `mod` 3 /= 0, x `mod` 5 /= 0]
       in
       sum nums `div` length nums

edited Aug 9, 2013 at 20:11

answered Aug 7, 2013 at 19:39

Jan Christiansen

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3 Comments

Borgleader Over a year ago

you have a typo, numbs should be nums

chirlu Over a year ago

Actually, I'd consider it an error if a function to compute the mean silently truncated the result.

user1786327 Over a year ago

Thanks for your help, I realised I had those errors too after I added the "in".

DiegoNolan · Accepted Answer · 2013-08-07 19:30:51Z

2

you need in

mean :: Int
mean = let nums = [x|x <- [1,3..1999], x 'mod' 3 != 0, x 'mod' 5 != 0]
       in sum nums/length nums

answered Aug 7, 2013 at 19:30

DiegoNolan

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2 Comments

user1786327 Over a year ago

Thanks, that helped, I had to fix a few other things too. I was using the wrong quotes for mod and the not equals operator is /= instead != and I had to change the / symbol on the last line to div. It works fine now

DiegoNolan Over a year ago

@user1786327 You might want to use fromIntegral to convert to a rational or floating point type. A mean of integers isn't really an integer.

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