Question

A simple question:
given the definitions, (From Haskell SOE)

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

and:

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

it seems that these two constructs are the same:

do let x = e1
   e2

and

do x <- e1
   e2

both evaluate e1, bind it to e2, and then evaluate e2.

Yes?

Answer 1

Let’s do a simple example in the Maybe monad:

foo = do
        let x = Just 1
        return x

and

bar = do
        x <- Just 1
        return x

Desugaring both, we get

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

For reference, I am using the translation from section 3.14 of the Haskell 2010 Report.