On 2 July 2010 01:59, prad wrote:

i'm trying to sort the assignment concept out in my mind. as i understand it so far,

<- is for IO

Not really :) <- is for working with monads. It is just syntactic sugar that makes using monads a bit more pleasant

so you can do something like someIOVar <- readFile filename this will be of type IO String

which is different from String as in

Yes, because underneath what is really going on is the compiler is changing that to readFile fileName >>= \x -> ... 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?

Not that you should be using before leaning how monads work ;) But there is unsafePerformIO

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)

in ghci you are *inside* the IO monad. Hence the need for let bindings

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)

actually this won't work, unless somefunction has type IO String -> IO a. You would need somefunction =<< readFile fn and that would have type IO a assuming somefunction returns something of type a

-- In friendship, prad

... with you on your journey Towards Freedom http://www.towardsfreedom.com (website) Information, Inspiration, Imagination - truly a site for soaring I's _______________________________________________ Beginners mailing list Beginners@haskell.org http://www.haskell.org/mailman/listinfo/beginners

You definitely need to read up on monads and the do notation. All will become clearer :) Ben

prad 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?

I think, you're building up the wrong intuition here. Let me try to correct it. First of all that neat left-arrow belongs to the 'do' notation. On the right hand side of the arrow you place an IO computation. On the left hand side you put a name for its result. This is not an assignment. You just give the result of the computation a name. If the computation is of type 'IO a', then that result is of type 'a'. Example: do line <- getLine putStr "You entered: " putStrLn line getLine is a computation of type IO String, so 'line' is of type String. There is no conversion involved, because actually there is no "running" involved at all. You just describe what happens, /when/ the 'main' computation is run. As you can see from its type, it's also an IO computation: main :: IO () Note that others have already noted that the 'do' notation as well as the '<-' operator, which belongs to it, is more general than that. This is true, but you can safely ignore that for now, until you have understood the general concept.

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 its raw form there are no mutable variables in Haskell. Technically there are, but from a high level language perspective there is no mutation. Whenever an equals sign is involved, this is just name giving, not an 'assignment' in the usual sense. Example: sum x y = x + y This gives 'x + y' a name, namely 'sum x y'. This really is an equation, so whenever you write 'x + y', you can safely replace it by 'sum x y' and vice versa. The same holds for 'let' and 'where', but unlike a top level definition they are scoped. You can view GHCi's command line as one 'do' block, which gets executed as you type it. That's why you need 'let'. You can't write top level definitions in GHCi. And the difference between '<-' and '=' is that '<-' gives the result of an IO computation a name, so you can refer to it. '=' just gives a value a name, where a value can be a function, too. In a 'let' construct, both sides of the equation have the same type. In a '<-' construct the right side has type 'IO a' and the left side has type 'a'. Right: let a = 4 Right: x <- getLine print (read x + 1) Now a few wrong ones with explanations: Wrong: putStrLn getLine Reason: putStrLn has the type 'String -> IO ()', so it expects a String, but getLine is of type 'IO String', not 'String'. Wrong: let x = getLine This doesn't produce an error by itself, but remember that both sides of an equation must have the same type. So x is of type 'IO String' now. Wrong: y <- x^2 The right hand side of '<-' must be of type 'IO a', but 'x^2' is of type (say) 'Integer'. This doesn't fit. I hope, this helps. Greets, Ertugrul -- nightmare = unsafePerformIO (getWrongWife >>= sex) http://blog.ertes.de/