2
Variable not in scope

For every piece of code that I wrote, I get ''variable not in scope''. (for example :
length1 :: [a] -> Int
length1 l | l==[] =0
| otherwise = 1 + length1 tail l
)
some of the other solutions i tried are written by my teaher.

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3

For one, you don't need that trailing parenthesis. Second, you need to add some parentheses because function calling has high precedence. Third, you need to add a type constraint for A if you are going to check if it's empty in this way. Also, I'd implement this with a foldl.

1

@ThatSmart Let me know if you need more help.