[Haskell-cafe] Couple of questions about *let* within *do*

[wren ng thornton]

michael rice wrote:
> OK, then there's also an implicit *in* after the *let* in this code. Must the implicit (or explicit) *in* actually use the calculated value(s)?
> 
> And, the monad can "continue on" after the *let* (with or without the *in*) as below, i.e., the *let* needn't be the last statement in the *do*?
> 


More specifically, there is an implicit "in do". So given some code,

     foo = do
         something
         let x = bar
             y = baz
         thingsome
         somesome

First there's the insertion of braces and semicolons implied by the 
layout rules.

     foo = do
         { something
         ; let { x = bar
               ; y = baz
         }; thingsome
         ; somesome
         }

Then we desugar the let-do notation,

     foo = do
         { something
         ; let { x = bar
               ; y = baz
         } in do
         { thingsome
         ; somesome
         }}

Or with prettier typesetting,

     foo = do
         { something
         ; let
             { x = bar
             ; y = baz
           } in do
             { thingsome
             ; somesome
             }
         }

and finally we can desugar do notation into (>>=), (>>), and fail.

-- 
Live well,
~wren