Folding, function composition, monads, and lazines

问题:

I am puzzled. I can write this:

import Control.Monad

main = print $ head $ (foldr (.) id [f, g]) [3]
  where f = (1:)
        g = undefined

and the output is 1. That makes sense, because it reduces to:

main = print $ head $ ((1:) . undefined . id) [3]
main = print $ head $ (1:) ((undefined . id) [3])
main = print $ head $ 1 : ((undefined . id) [3])
main = print $ 1

But if I use a vaguely similar monadic technique, it doesn't work the same:

import Control.Monad

main = print $ (foldr (<=<) return [f, g]) 3
  where f = const Nothing
        g = undefined

This hits prelude.Undefined. Which is odd, because I would expect it to reduce:

main = print $ ((const Nothing) <=< undefined <=< return) 3
main = print $ return 3 >>= undefined >>= (\_ -> Nothing)
main = print $ Nothing -- nope! instead, undefined makes this blow up

However, flipping the order of composition:

import Control.Monad

main = print $ (foldr (>=>) return [f, g]) 3
  where f = const Nothing
        g = undefined

does accomplish the expected short-circuiting and produces Nothing.

main = print $ (const Nothing >=> undefined >=> return) 3
main = print $ (const Nothing 3) >>= undefined >>= return
main = print $ Nothing >>= undefined >>= return
main = print $ Nothing

I suppose comparing the two approaches might have been comparing apples and oranges, but can you explain the difference? I thought that f <=< g was the monadic analogue to f . g, but they are apparently not as analogous as I thought. Can you explain why?

回答1:

It depends on which monad you're working with, and how its (>>=) operator is defined.

In the case of Maybe, (>>=) is strict in its first argument, as Daniel Fischer explained.

Here are some results for a handful of other monads.

> :set -XNoMonomorphismRestriction
> let foo = (const (return 42) <=< undefined <=< return) 3
> :t foo
foo :: (Num t, Monad m) => m t

Identity: Lazy.

> Control.Monad.Identity.runIdentity foo
42

IO: Strict.

> foo :: IO Integer
*** Exception: Prelude.undefined

Reader: Lazy.

> Control.Monad.Reader.runReader foo "bar"
42

Writer: Has both a lazy and a strict variant.

> Control.Monad.Writer.runWriter foo
(42,())
> Control.Monad.Writer.Strict.runWriter foo
*** Exception: Prelude.undefined

State: Has also both a strict and a lazy version.

> Control.Monad.State.runState foo "bar"
(42,"*** Exception: Prelude.undefined
> Control.Monad.State.Strict.runState foo "bar"
*** Exception: Prelude.undefined

Cont: Strict.

> Control.Monad.Cont.runCont foo id
*** Exception: Prelude.undefined

回答2:

The bind for Maybe is strict in the first argument.

Just v >>= f = f v
Nothing >>= f = Nothing

So when you try

Just v >>= undefined >>= \_ -> Nothing

you hit

undefined v >>= \_ -> Nothing

and the implementation needs to find out whether undefined v is Nothing or Just something to see which equation of (>>=) to use.

On the other hand,

Nothing >>= undefined

determines the result without looking at the second argument of (>>=).