[Haskell-beginners] the role of assignments

[Jordan Cooper]

<- isn't just for IO; it's part of "do" notation's syntactic sugar. It
is actually (in your first example) passing the unwrapped result
(String; not IO String) of ``readFile filename" to an anonymous
function.

I'd recommend reading ch. 14 of Real World Haskell (
http://book.realworldhaskell.org/read/monads.html )

On 7/1/10, prad <prad at towardsfreedom.com> wrote:
> i'm trying to sort the assignment concept out in my mind.
> as i understand it so far,
>
> <- is for IO
> so you can do something like
> someIOVar <- readFile filename
> this will be of type IO String
>
> which is different from String as in
>
> let someStrVar = "this is a string"
>
> to work with an IO String you need to convert it into a String which
> seems to automatically happen inside a do structure as in:
>
> main = do
>     tmplX <- readFile "zztmpl.htm"
>     navbx <- readFile "zznavb.htm"
>     let page = U.replace tmplX "NAVX" navbx
>
> are there some other ways to make IO String into String?
>
>
>
> also, it seems that assignment is different for the '=' in a program vs
> ghci for functions:
>
> sum x y = x + y (program)
> vs
> let sum x y = x + y (ghci)
>
> but not for strings and other things because you must always preface
> assignment of values with 'let':
>
> let a = 4
>
> i suppose the let is there for variable assignments because such things
> necessitate a change of state and i've read that this is not desirable
> in functional programming - you have to work a little harder to do
> assignment than languages which just allow
> a = 4
> b = 5
> c = 6
> etc
>
> in haskell, is it preferable to do something like this:
>
> var <- readFile fn
> someFunction var
>
> or someFunction (readFile fn)
>
>
> --
> In friendship,
> prad
>
>                                       ... with you on your journey
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