Question

In a Haskell tutorial I ran across the following code:

do [...]
  let atom = [first] ++ rest
  return $ case atom of

Note that the let expression does not have an in block. What is the scope of such a let expression? The next line?

Answer 1

Simply put, it’s scoped “from where it’s written until the end of the do“.

Note that within a do statement, let is handled differently.

According to http://www.haskell.org/haskellwiki/Monads_as_computation#Do_notation , it is interpreted as follows:

do { let <decls> ; <stmts> }
  = let <decls> in do { <stmts> }