load and use code file in haskell

Question

I'm just starting to learn haskell and i dont really understand how to use files i created with an ordinary editor in the GHCi interpreter...

This is my file list-comprehension.hs

main = do
let substantive = [" Student", " Professor", " Tutor"]
let adjektive = ["fauler", "fleissiger", "hilfreicher"]
let tupel = [a ++ s | a <- adjektive, s <- substantive]
return (tupel)

When I load the file in GHCi it works alright, but then I cant actually use it. So when I try to execute tupel, it wont work.

Prelude> :load list-comprehension.hs
[1 of 1] Compiling Main             ( list-comprehension.hs, interpreted )
Ok, modules loaded: Main.

*Main> tupel   
<interactive>:3:1: error: Variable not in scope: tupel

This also happens when I try to get the other variables. I have researched this a lot, but I cant find out whats wrong with my file or how this generally works... I'm not at all sure about the "main = do" and the "return" part, but this is the only beginning and end that doesnt produce a parse error when loading .

if you want to use the definitions in GHCI, define them outside of the main block, without the let statement. — karakfa
– karakfa, Commented May 1, 2017 at 16:33

Lazersmoke · Accepted Answer · 2017-05-01 16:49:46Z

1

GHCi only has the top level definitons from a file in scope. Try this:

main :: IO ()
main = print tupel

substantive :: [String]
substantive = [" Student", " Professor", " Tutor"]

adjektive :: [String]
adjektive = ["fauler", "fleissiger", "hilfreicher"]

tupel :: [String]
tupel = [a ++ s | a <- adjektive, s <- substantive]

answered May 1, 2017 at 16:49

Lazersmoke

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2 Comments

BadWolf Over a year ago

Thank you very much! I think I understand what you did, ill read up on this ;)

Lazersmoke Over a year ago

Just to be clear, the type signatures aren't mandatory, but you should definitely use them to help the people who read your code, including yourself.

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