Re: [Haskell-beginners] IO and ghci

Thank you Friedrich, that really does help. At some point I became confused as I’m *sure* that I tried file <- readFile “poem.txt” in ghci and it didn’t work! Clearly it does. :) Thanks again. Mike

On 19 Apr 2015, at 11:03, Friedrich Wiemer wrote:

Hi Mike as you perhaps have noticed, you use slightly different notations here:

...

file <- readFile "poem.txt"

...

let f = readFile “poem.txt”

You can run both versions in ghci, and if you take a look at the type signatures: λ> file <- readFile "poem.txt" λ> :t file file :: String

λ> let f = readFile "poem.txt" λ> :t f f :: IO String

You'll notice that f has type "IO String", whereas file only has type "String".

So, to answer your second question: you can progress by using the "<-" notation, as in your haskell source code.

To answer the first one: I'm not sure about ghci command prompt, but I would expect, that it is equivalent to "main = do ...". That is, the type error you got, is not due to ghci and main = do are different, but because you used to different types.

To understand the "<-" notation, the example from Learn you a Haskell was quite good for me: Try to see types like "IO String" as an action or box, which can result in a string. You can take the inner type out of the box, by using "<-".

Hope that helps,

cheers, Friedrich

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