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]
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.
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]
-- ^^^ ^^^
