Haskell - parse error on input

Question

I have just started using Haskell interpreter.

Can anyone tell me what's wrong with this code:(for Fibonacci)

fib :: Int -> Int
fib n
|n==0 =0
|n==1 =1
|n>1  =fib(n-2) + fib(n-1)

I am getting this error message:

 fib.hs:3:1: parse error on input `|'
 [1 of 1] Compiling Main             ( fib.hs, interpreted )
 Failed, modules loaded: none.

Ørjan Johansen · Accepted Answer · 2014-05-03 11:01:52Z

4

Haskell is indentation sensitive. In particular the |'s need to be indented further than the function name:

fib :: Int -> Int
fib n
  |n==0 =0
  |n==1 =1
  |n>1  =fib(n-2) + fib(n-1)

Also, it is more idiomatic to use pattern matching for comparing with a constant:

fib :: Int -> Int
fib 0 =0
fib 1 =1
fib n
  |n>1  =fib(n-2) + fib(n-1)

answered May 3, 2014 at 11:01

Ørjan Johansen

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2 Comments

Do we need the guard in the last declaration?

@ssm Not strictly, but it gives a faster error on attempts to call it with negative numbers. I really just left it in because it was in the original.

chi · Accepted Answer · 2014-05-03 11:02:36Z

1

You have to indent the next lines at least one space, e.g.:

fib :: Int -> Int
fib n
 |n==0 =0
 |n==1 =1
 |n>1  =fib(n-2) + fib(n-1)

answered May 3, 2014 at 11:02

chi

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