Haskell 中的 >> 符号是什么意思

Question

I was reading the Guestbook example for Happstack and noticed the >> symbol which I didn't see before in the textbooks I studied to learn Haskell (for instance see line 23). What is it?

I could not find it in Google because it ignores the >> totally (Bing does not but comes up with tons of non-related results).

Answer 1

In do-notation

a >> b >> c >> d

is equivalent to

do a
   b
   c
   d

(and similarly a >>= (b >>= (c >>= d)) is equivalent to

do r1 <- a
   r2 <- b r1
   r3 <- c r2
   d r3

Answer 2

Hayoo recognises this kind of operator: http://holumbus.fh-wedel.de/hayoo/hayoo.html

(>>) is like (>>=) , in that it sequences two actions, except that it ignores the result from the first one.

Answer 3

At the ghci command prompt, you can type:

:info >>

And get a result like:

class Monad m where
...
(>>) :: m a -> m b -> m b
...
        -- Defined in GHC.Base
infixl 1 >>

From there, you can just take a look at the source code to learn more.

And just for the sake of answering your question:

k >> f = k >>= \_ -> f

Answer 4

From Hackage, >> is described as:

"Sequentially compose two actions, discarding any value produced by the first, like sequencing operators (such as the semicolon) in imperative languages."

I think a good example is printing two strings sequentially using >> . Open GHCI and type the following:

putStr "Hello " >> putStrLn "World"

This is equivalent to the do notation:

do putStr "Hello "
   putStrLn "World"

Answer 5

I'm no Haskell expert, but >> is an operator that is used for working with monads, which are an unusual feature that (among many other things) enable imperative-style programming in Haskell. There are many tutorials available on monads; here's one good one .

Essentially, a >> b can be read like "do a then do b , and return the result of b ". It's similar to the more common bind operator >>= .