CODEWITHSUNDEEP
haskellghci
randominstanceOfLivingThing
randominstanceOfLivingThing

Reputation: 18642

Getting Multiline haskell function to work in ghci

I created this haskell function to remove odd numbers from a list and was trying in ghci. I keep getting the error below though I have enabled the multiline mode and used 4 spaces for indentation. 

Prelude> :set +m
Prelude> let removeOdds nums =
Prelude|     if null nums
Prelude|     then []
Prelude|     else
Prelude|         if (mod (head nums)/2) == 0
Prelude|         then (head nums) : (removeOdds(tail nums))
Prelude|         else removeOdds(tail nums)
Prelude|

:11:5: parse error (possibly incorrect indentation or mismatched brackets)

I read this page about the common mistable newbie's will make and I changed my code as below

Prelude> let removeOdds nums =
Prelude|     do if null nums
Prelude|         then []    
Prelude|         else 
Prelude|           do if mod((head nums)/2) == 0
Prelude|               then head nums: removeOdds(tail nums)   
Prelude|               else removeOdds(tail nums)
Prelude| 

<interactive>:47:5:
    parse error (possibly incorrect indentation or mismatched brackets)

Now I ended with a new error. It appears the indentation is a tough thing to get around in haskell.

Upvotes: 2

Views: 1087

Answers (3)

R B
R B

Reputation: 1107

I use curly braces in GHCi when defining multiline functions. They allow you to just paste in a big block of Haskell code like you would usually write.

λ :{
| let
| -- type or paste here, e.g.,
| removeOdds :: Integral a => [a] -> [a]
| removeOdds nums =
|   if null nums
|   then []
|   else -- ...
| :}
λ removeOdds [1,2,3,4]
[2,4]

Upvotes: 3

ErikR
ErikR

Reputation: 52029

Your if statement needs to be indented at least one more space:

Prelude> let removeOdds nums =
Prelude|      if null nums

Complete example:

Prelude> let removeOdds nums =
Prelude|      if null nums
Prelude|      then []
Prelude|      else
Prelude|          if (mod (head nums) 2) == 0
Prelude|          then (head nums) : (removeOdds(tail nums))
Prelude|          else removeOdds(tail nums)
Prelude|
Prelude>

Upvotes: 4

Steven Shaw
Steven Shaw

Reputation: 6249

It may also be good to note that when you aren't in the REPL and therefore don't require let, then you don't need so much indentation:

removeOdds nums =
  if null nums
  then []
  else
    if (mod (head nums) 2) == 0
    then (head nums) : (removeOdds(tail nums))
    else removeOdds(tail nums)

Upvotes: 1

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