Haskell <- in do notation

Question

It's well known that <- in do blocks are just syntactic sugar for >>=, but is <- defined anywhere in the Haskell source or is it just syntactic construction that is part of language grammar and thus the parser just replaces every <- with appropriate form of >>=?

Answer 1

The Do Expression is part of the Language.

From the Haskell Language Report 2010:

3.14 Do Expression

lexp  →   do { stmts }            (do expression)
stmts →   stmt1 … stmtn exp [;]   (n ≥ 0)  
stmt  →   exp ;
      |   pat <- exp ;
      |   let decls ; 
      |   ;                       (empty statement)

A do expression provides a more conventional syntax for monadic programming. It allows an expression such as

 putStr "x: "    >>  
 getLine         >>= \l ->     
 return (words l)

to be written in a more traditional way as:

 do putStr "x: "  
    l <- getLine  
    return (words l)

As indicated by the translation of do, variables bound by let have fully polymorphic types while those defined by <- are lambda bound and are thus monomorphic.

see Do Expression of the Haskell Language Report.