Haskell - parsing error on input

Question

I am new to haskell and stuck with the input parse error on '=' I was trying a function to check if a number is prime Here is the function -

isprime x
| x<=1 = "not prime"
| x == 2 = "yes"
| a = floor (sqrt x)
  lis = [2..a]
  divbylis (headlis:taillis)
    | length taillis <=0 = "yes it is prime"
    | x 'mod' headlis == 0 = "No not prime"
    | otherwise = divbylis taillis**

and it shows the error -

first.hs:58:15: error:
parse error on input ‘=’
Perhaps you need a 'let' in a 'do' block?
e.g. 'let x = 5' instead of 'x = 5'

line 58 is the line with declaration of lis = [2..a]**

You write a = floor (sqrt 5), which makes no sense since Haskell expects a guard. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jul 13, 2018 at 9:26
The | a = floor (sqrt x) line is weird - what's a? I think you meant to write a where at the end of that line, but that won't fix the problem you're having here. — Cubic
– Cubic, Commented Jul 13, 2018 at 9:29
Just using the common code of checking prime , creating a list of numbers from 2 to sqrt of the number and dividing by each of them — jatin
– jatin, Commented Jul 13, 2018 at 11:47
@jatin: but what you do is using imperative style code into a functional/declarative language. One of the ideas behind Haskell is to move away from that imperative approach. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jul 13, 2018 at 11:52

sshine · Accepted Answer · 2018-07-13 10:50:22Z

2

Here is some feedback:

You posted some code and a syntax error, but you didn't ask a question. Presumably your question is "Why doesn't this code work?" but that's actually not a good question. How do I ask a good question?
Your function is a predicate, but it returns a String. Consider returning a Bool instead.
Give your function a type signature:
```
isprime :: Int -> Bool
isprime ...
```
Your main syntax error is due to indentation; your guards are at the top-level. Write at least one space before the guards:
```
isprime :: Int -> Bool
isprime x
  | x <= 1 = False
  | x == 2 = True
  | ...
```

Use pattern matching when appropriate:

isprime :: Int -> Bool
isprime 1 = False
isprime 2 = True
isprime n = ...

I'm not reaaally sure what goes on at the "lis = [2..a]" and "divbylis (headlis:taillis)" part. It doesn't look like Haskell code. Is that supposed to be a helper function?
If writing a prime function is too difficult, then try writing simpler numeric, recursive functions like factorial, power, fibonacci, collatz. If you want to see how others have done it, see: Determining if a given number is a prime in haskell, Is there a better way to write the isPrime function?

answered Jul 13, 2018 at 10:50

sshine

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1 Comment

jatin Over a year ago

Thanks, headlis is just the first element of the list and taillis is the remaining list. My bad , it was my first question on stackoverflow and also thanks for your concerns

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