Expressing do block using only monadic bind syntax

问题:

As far as I know, do blocks in Haskell are just some kind of syntactic sugar for monadic bind operators. For example, one could convert

main = do f <- readFile "foo.txt"
          print f
          print "Finished"

to

main = readFile "foo.txt" >>= print >> print "Finished"

Can all do blocks be converted to bind syntax? What about, for example, this block where f is used multiple times:

main = do f <- readFile "foo.txt"
          print $ "prefix " ++ f
          print $ f ++ " postfix"

Assuming we are in the IO monad, it is not possible to simply execute the readFile computation twice. How can this example (if possible at all) expressed using only bind syntax?

I think using Control.Monad is no solution, because it internally uses do blocks.

I think it it's possible to express this using arrows (using &&&) -- maybe this is a case where only arrows can be used as a generalization of monads?

Note that this question is not about the special examples above but about the general case of the result of a computation being used multiple times in monadic expressions like print.

回答1:

Yes, all of them can be converted to bind syntax; in fact, they are converted internally by the compiler.

I hope this translation of your example gives you the hint:

main = readFile "foo.txt" >>= \f ->
       (print $ "prefix " ++ f) >>
       (print $ f ++ " postfix")

回答2:

The Report gives a full translation from do syntax into kernel Haskell:

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}

The ellipsis "..." stands for a compiler-generated error message, passed to fail, preferably giving some indication of the location of the pattern-match failure; the functions >>, >>=, and fail are operations in the class Monad, as defined in the Prelude; and ok is a fresh identifier.

So your example translates this way:

do f <- readFile "foo.txt"
   print $ "prefix " ++ f
   print $ f ++ " postfix"
=
let ok f = do print $ "prefix " ++ f
              print $ f ++ " postfix"
    ok _ = fail "..."
in readFile "foo.txt" >>= ok
=
let ok f = (print $ "prefix " ++ f) >> do print $ f ++ " postfix"
    ok _ = fail "..."
in readFile "foo.txt" >>= ok
=
let ok f = (print $ "prefix " ++ f) >> (print $ f ++ " postfix")
    ok _ = fail "..."
in readFile "foo.txt" >>= ok

This version has no do blocks, but doesn't look very natural. But we can apply equational reasoning and whatever optimizations we know. So, for example, observing that the ok _ = fail "..." clause is dead code, we could inlike ok like so:

 =
 readFile "foo.txt" >>= \f ->
 (print $ "prefix " ++ f) >>
 (print $ f ++ " postfix")

All do blocks can be mechanically translated into code without do in this way.