The desugaring is simpler with the current setup:
do { e }
=> e
do { let p = e; STMTS }
=> let p = e in (do { STMTS })
do { e; STMTS }
=> e >> (do { STMTS })
do { p <- e; STMTS }
=> e >>= \x -> case x of { p -> (do { STMTS }) ; _ -> fail "pattern match
failure" }
[x is a fresh variable]
My guess is that >> is infixl because
(1) m >>= f >>= g should make sense
(2) >> should match fixity and precedence with >>=
On Tue, Feb 14, 2012 at 9:50 PM, Michael Baikov
Most docs ([1], [2]) about do-notation syntactic sugar tends to describe following expressions as equivalent:
"do { a; b; c }" and "a >> b >> c", but they are not: first one gets de-sugared into "a >> (b >> c)", second one is equivalent to "(a >> b) >> c", because (>>) is declared using infixl.
This should not be a problem, monadic law of Associativity states that "(m >>= f) >>= g ≡ m >>= (\x -> f x >>= g)", but this leads to generating different Core output and may lead to different performance (and it does, do { Just 4 ; Just 4 ... } is about 2% faster than Just 4 >> Just 4 >> ... if compiled with -O0, but 13% slower when compiled with -O11)
This also leads to lots of fun when your monad breaks Associativity law :)
Is there any reasons except for those 13% speed gain for this?
[1]: http://en.wikibooks.org/wiki/Haskell/do_Notation [2]: http://book.realworldhaskell.org/read/monads.html#monads.dot
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