[Haskell-beginners] Re: question on layout

[Maurí­cio]

>>> do
>>>      a <- x
>>>      let b = a
>>>      y b
>>>      z
>>>
>>> expands to
>>>
>>> do {a <- x ; let {b = a} in do {y b >> z}}

>> I'm curious as to where the second `do' came from?

> Well, the above translation isn't quite correct, the second 'do'
> wouldn't come until later.  The point is that 'do { let x = y; foo }'
> translates to 'let x = y in do { foo }'.

Exatly, I should have checked better that example.

I just thought it worth to show how 'let' translates
in a 'do' expression, because it caused me a lot of
trouble when I learned Haskell. Since I had read that
'do' expressions are supposed to chain (Monad m) => (m a)
elements, I assumed from this use of 'let' that:

(WARNING: WRONG ASSUMPTIONS!)

* 'let a = b' has type (Monad m) => (m x), x the type
   of a and b.

* Since 'let' is used in let expressions and also in
   do expressions, either Haskell allows redefinition
   of reserved keywords or 'let' is not a reserved
   keyword and there's some way in Haskell to define
   constructs like 'let ... in ...' using more basic
   primitives.

* Haskell has some kind of overloading allowing one
   word to be used in unrelated contexts.


That's of course completely nonsense, but I would be
happy if I could avoid others from running into this
kind of misunderstanding.

Best,
Maurício