What is <-

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michael rice

8 Aug 2010 8 Aug '10

9:36 a.m.

What is <- ? Couldn't find anything on Hoogle. 1) main = do x <- getLine -- get the value from the IO monad putStrLn $ "You typed: " ++ x 2) pythags = do z <- [1..] --get the value from the List monad? x <- [1..z] y <- [x..z] guard (x^2 + y^2 == z^2) return (x, y, z) From: http://en.wikibooks.org/wiki/Haskell/Syntactic_sugar Do and proc notation Sweet Unsweet Monadic binding do x <- getLIne getLine >>= \x -> putStrLn $ "You typed: " ++ x putStrLn $ "You typed: " ++ x So, Example 2 desugared becomes... [1..] >== \z -> ? Michael

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Henning Thielemann

8 Aug 8 Aug

9:38 a.m.

On Sun, 8 Aug 2010, michael rice wrote:

...

So, Example 2 desugared becomes...

[1..] >== \z -> ?

Yes, [1..] >>= \z -> ...

michael rice

10:21 a.m.

getLine >>= \x -> -- x is a string at this point [1..] >>= \x -> -- x is WHAT at this point? MIchael --- On Sun, 8/8/10, Henning Thielemann wrote: From: Henning Thielemann Subject: Re: [Haskell-cafe] What is <- To: "michael rice" Cc: haskell-cafe@haskell.org Date: Sunday, August 8, 2010, 9:38 AM On Sun, 8 Aug 2010, michael rice wrote:

...

So, Example 2 desugared becomes...

[1..] >== \z -> ?

Yes, [1..] >>= \z -> ...

Christopher Done

10:26 a.m.

On 8 August 2010 16:21, michael rice wrote:

...

getLine >>= \x -> -- x is a string at this point

[1..] >>= \x -> -- x is WHAT at this point?

Num n => n A number from the list.

Markus Läll

10:38 a.m.

Hi Michael, although I never used it myself, lists seem strange in the way that when combining list monads, then all the values go through the chain one by one -- x will be 1 first, then 2, then 3 and so on.. Try it out, to see. (I think the result is then also a list of all combinations of results.) On Sun, Aug 8, 2010 at 5:26 PM, Christopher Done wrote:

...

On 8 August 2010 16:21, michael rice wrote:

...
getLine >>= \x -> -- x is a string at this point

[1..] >>= \x -> -- x is WHAT at this point?

Num n => n

A number from the list.

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

In shortyes, "do z <- ..; foo" desugars to "... >>= \z -> foo" The Haskell Report describes `do' notation in detail: http://www.haskell.org/onlinereport/exps.html#sect3.14 Real World Haskell describes its uses: http://book.realworldhaskell.org/read/io.html#io.bind On 8 August 2010 15:36, michael rice wrote:

...

What is <- ? Couldn't find anything on Hoogle.

1) main = do x <- getLine -- get the value from the IO monad putStrLn $ "You typed: " ++ x

2) pythags = do z <- [1..] --get the value from the List monad? x <- [1..z] y <- [x..z] guard (x^2 + y^2 == z^2) return (x, y, z)

From: http://en.wikibooks.org/wiki/Haskell/Syntactic_sugar

Do and proc notation

Sweet Unsweet Monadic binding do x <- getLIne getLine >>= \x -> putStrLn $ "You typed: " ++ x putStrLn $ "You typed: " ++ x

So, Example 2 desugared becomes...

[1..] >== \z -> ?

Michael

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Miguel Mitrofanov

9:42 a.m.

On 8 Aug 2010, at 17:36, michael rice wrote:

...

What is <- ? Couldn't find anything on Hoogle.

1) main = do x <- getLine -- get the value from the IO monad putStrLn $ "You typed: " ++ x

2) pythags = do z <- [1..] --get the value from the List monad? x <- [1..z] y <- [x..z] guard (x^2 + y^2 == z^2) return (x, y, z)

From: http://en.wikibooks.org/wiki/Haskell/Syntactic_sugar

Do and proc notation

Sweet Unsweet Monadic binding do x <- getLIne getLine >>= \x -> putStrLn $ "You typed: " ++ x putStrLn $ "You typed: " ++ x

So, Example 2 desugared becomes...

[1..] >== \z -> ?

[1..] >>= \z -> [1..z] >>= \x -> [x..z] >>= \y -> guard (x^2 + y^2 == z^2) >> return (x, y, z) Isn't that kinda obvious?

...

Michael

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