[Haskell-beginners] IO and ghci

[Mike Houghton]

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 <friedrichwiemer at gmail.com> 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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