re-write this example without do

Question

I am reading that Haskell's do notation is quite harmful. Since I am still learning Haskell, I would like to not make bad habits. I am learning a library called Reflex and is an example:

import Reflex.Dom

main = mainWidget $ el "div" $ do
  t <- textInput def
  dynText $ _textInput_value t

I am reading that do notation has to do with the presence of monads such as IO and has to do with <- and $ operators.

How could I write these few lines without do ?

If you support do... could you explain it's use in Haskell and this example?

do notation is not harmful. in fact, do-notation has been expanded to applicatives, so we're encouraging its use even more. — ErikR, Jul 05 '16 at 23:07
Every single answer on the question you linked says that `do` notation is useful and should not be avoided. — interjay, Jul 05 '16 at 23:11

score 2 · Answer 1 · answered Jul 05 '16 at 23:08

do-notation is pure syntactic sugar. It translates into calls to >>= in a pretty simple way. Quoting the Haskell Report:

Translation: Do expressions satisfy these identities, which may be used as a translation into the kernel, after eliminating empty stmts:
do {e}    =   e
do {e;stmts}  =   e >> do {stmts}
do {p <- e; stmts}    =   let ok p = do {stmts}
    ok _ = fail "..."
  in e >>= ok
do {let decls; stmts} =   let decls in do {stmts}

This means your example would look like this:

main = mainWidget $ el "div" $
  textInput def >>= $ \t ->
  dynText $ _textInput_value t

There's nothing harmful about do-notation. I don't know where you got that idea from! It's literally a different way of writing the same code.

re-write this example without do

1 Answers1