Haskell Lambda Functions

Question

I am new to Haskell and programming in general. I am trying to write a lambda function that return a value squared

(\x  -> x * x)

this is the code I have written. When i try to compile I get this error Parse error: naked expression at top level Perhaps you intended to use TemplateHaskell

I have googled it I cannot fund a solution

You should assign this to a function, for example square = \x -> x * x. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Jun 25, 2021 at 11:34
Thanks that works run if I want to print this I used this code — K Lawson
– K Lawson, Commented Jun 25, 2021 at 11:39

willeM_ Van Onsem · Accepted Answer · 2021-06-25 11:51:30Z

1

You should assign this to a identifier, since \x -> x * x as program does not make much sense: you define a function, but then you throw that function away.

You thus can implement a function:

mysquare :: Num a => a -> a
mysquare = \x -> x * x

then you can call the function when necessary. We can for example define a main with:

main :: IO ()
main = putStrLn (show (mysquare 5))

and then call main:

Prelude> main
25

edited Jun 25, 2021 at 11:51

answered Jun 25, 2021 at 11:36

willeM_ Van Onsem

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5 Comments

K Lawson Over a year ago

printsquare = putStrLn (show (mysquare 5)) does not print anything to the console ?

willeM_ Van Onsem Over a year ago

@KLawson: you defined a function, you did not call that function.

K Lawson Over a year ago

I am confused because I am new to haskell. I already tried this putStrLn (show (mysquare 5)) I get an 'errorParse error: naked expression at top level Perhaps you intended to use TemplateHaskell'. with the previous code it runs but it doesn't output anything

willeM_ Van Onsem Over a year ago

@KLawson: you should assign it to the main to start the program at that point, so main = putStrLn (show (mysquare 5)), then compile and run the compiled program.

K Lawson Over a year ago

Thank you this makes sense. And it actually helped to understand how Haskell works a bit better.

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