using where clause in haskell causing error?

Question

I am new in haskell trying to do a small task

get1th ( a , _, _ , _) = a
foo input = 
    where input = (a:_,b:_,c:_,d:_)
    if ( length (get1th input) == 2 ) 
        then permutations[2]
        else permutations[3]

I am getting error saying

 parse error on input `where'

please give me a hint

This is not how where is used. Take a look here: learnyouahaskell.com/syntax-in-functions#where — Rodrigo Taboada
– Rodrigo Taboada, Commented Oct 14, 2013 at 19:17

wit · Accepted Answer · 2013-10-15 19:22:59Z

3

where clause must be written at the end:

foo input = 
    if ( length (get1th input) == 2 ) 
        then permutations[2]
        else permutations[3]
    where (a:_,b:_,c:_,d:_) = input

UPDATED

It is also require to swap to (a:_,b:_,c:_,d:_) = input, reason - we want to extract values, but not to redefine input

edited Oct 15, 2013 at 19:22

answered Oct 14, 2013 at 19:35

wit

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3 Comments

Daniel Wagner Over a year ago

...but this is still really broken.

viorior Over a year ago

@user2277918 : correct last line to where (a:_,b:_,c:_,d:_) = input

pat Over a year ago

The where clause seems totally unnecessary since none of the extracted values are used, and the failure to pattern match will result in an exception. At best, it restricts the inferred type of input to a 4-tuple of lists of something... Also, the permutations[2] looks like array indexing, but it is actually applying the permutations function to the list containing a single 2, which does not have a very interesting result.

Guiraldelli · Accepted Answer · 2013-10-15 00:08:18Z

1

As @wit has pointed out, where should be used in the end of expression. Why? Because:

it is not a let;
because it is related with all the previous context (block) of the function.

If you want to define an alias fronthand, you should use the let expression.

For more information about the differences and advantages of them, see the Let vs. Where.

answered Oct 15, 2013 at 0:08

Guiraldelli

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