[Haskell-beginners] File I/O: reading from, writing to, same file in one function

[Geoffrey Bays]

Thanks, Chaddai. It seems quite obvious once you explain it. And I now know
that ghci :t is my friend indeed.

Geoffrey

On Thu, Feb 5, 2015 at 4:55 PM, Chaddaï Fouché <chaddai.fouche at gmail.com>
wrote:

> On Thu, Feb 5, 2015 at 10:43 PM, Geoffrey Bays <charioteer7 at gmail.com>
> wrote:
>>
>> The problem also with using withFile and a lambda is that in my infinite
>> Haskell beginnerness, I do not know how to get the
>> contents out of the lambda for use later for the second try of withFile
>> in WriteMode.
>>
>> As in LYAH
>>
>>    1.     withFile "girlfriend.txt" ReadMode (\handle -> do
>>    2.         contents <- hGetContents handle
>>    3.         putStr contents)
>>
>> how to retrieve contents for later use? Scope of contents variable in
>> inside lambda only it would appear.
>>
>
> Well withFile type is "FilePath -> Mode -> (Handle -> IO a) -> IO a", note
> that "IO a", this "a" is a type variable that can be any type, this means
> that the action you do in your lambda may return any type, and the type
> returned by the whole withFile action is also "a", since this can be
> anything, withFile can't invent it, so this must be the same thing that the
> action in your lambda returned. Thus you can return the content you wished
> for :
>
>   contents <- withFile fileName ReadMode $ \h -> do
>     contentsInside <- hGetContents handle
>     evaluate (length contentsInside) -- still the same, you have to
> evaluate the whole contents now since withFile will close the handle as
> soon as you exit your lambda, use a strict variant of hGetContents to avoid
> this line
>     return contentsInside
>
> Note this is convoluted, using a strict variant of hGetContents would
> allows you to just go : contents <- withFile fileName ReadMode $ \h ->
> hGetContents h
> Text for instance provide such a strict variant in Data.Text.IO (but you
> should then just use its strict variant of readFile which is exactly
> equivalent to what I just wrote).
>
> --
> Jedaï
>
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