CODEWITHSUNDEEP
haskell
Chalupa
Chalupa

Reputation: 367

getting parse error on input '=' error (haskell)

my function append takes a list of lists [[a], [b,c], [d,e]] and returns a single list [a,b,c,d,e]. I wrote this in a file so i didn't have to use "let" but i still get parse error on input '='. can anyone help? thanks

append :: [[a]] -> [a]
append [[a]] = [ a | len = length a, n = 1, head a ++ (a !! n) , n < len]

Upvotes: 0

Views: 92

Answers (2)

bheklilr
bheklilr

Reputation: 54068

You need let for len and n:

append [[a]] = [a | let len = length a, let n = 1, head a ++ (a !! n), n < len]

But this won't solve all of your problems, once the lets are added it doesn't typecheck, and [[a]] is probably not the pattern you want to use here. The pattern [[a]] will only match a list like [[1]], it won't match [], or [[1, 2]], or [[1], [2]].

You also have another problem that head a ++ (a !! n) should be an expression that returns a Bool, but in this case it's returning a list. Any "naked" expressions on the right side of the | in a list comprehension must evaluate to a Bool value.

If you're wanting to flatten a list of lists, I would suggest looking at the built-in concat function, which is actually defined using foldr. Folds can be tricky to learn, though, so I'll show an alternate definition using explicit recursion:

myconcat :: [[a]] -> [a]
myconcat [] = []
myconcat (x:xs) = x ++ myconcat xs

This is equivalent to the foldr definition, but hopefully it can be more instructive to how to solve this sort of problem.

Upvotes: 4

sepp2k
sepp2k

Reputation: 370367

You still need to use let to define local variables inside a list comprehension regardless of whether or not the code is in a file. The code being in a file only makes a difference for top-level definitions. Local definitions stay the same.

So the syntactically correct version of your code would be:

append :: [[a]] -> [a]
append [[a]] = [ a | let len = length a, let n = 1, head a ++ (a !! n) , n < len]
--                   ^^^                 ^^^

Upvotes: 0

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