Haskell Monadic 形式

Question

一个简单的问题:给定定义，(来自 Haskell SOE)

   do x — el; el\ ...; en 
    => el »= \x — do e2\ ...; en 

和:

do let decllist; el\...; en 
=> let decllist in do e2\ ...; en 

这两个结构似乎是相同的:

do let x = e1
   e2

和

do x <- e1
   e2

两者都评估 e1，将其绑定(bind)到 e2，然后评估 e2。

是吗？

Answer 1

让我们在 Maybe monad 中做一个简单的例子:

foo = do
        let x = Just 1
        return x

和

bar = do
        x <- Just 1
        return x

将两者脱糖，我们得到

foo = let x = Just 1 in return x    -- do notation desugaring
    = return (Just 1)               -- let
    = Just (Just 1)                 -- definition of return for the Maybe monad

bar = let ok x = return x in Just 1 >>= ok   -- do notation desugaring
    = let ok x = return x in ok 1   -- definition of >>= for the Maybe monad
    = return 1                      -- definiton of ok
    = Just 1                        -- definition of return for the Maybe monad

作为引用，我正在使用 translation from section 3.14 of the Haskell 2010 Report .