Intuitively it seems like the applicative expression:
(++) <$> getLine <*> getLine
should represent the same thing as the more traditional liftM2
expressions:
do { x <- getLine; y <- getLine; return ((++) x y) }
but it seems to me that you cant directly manipulate the first into
the second. I get:
do x2' <- getLine
x1 <- return ((++) x2')
x2 <- getLine
return (x1 x2)
the only way I can get from here to the liftM2 definition is if I
treat "x1 <- return ((++) x2')" as "let x1 = (++) x2", and then
allow it to be reordered after the second getLine. Then it is
straightforward to reduce to the liftM2 expression above.
It seems to me that this is a valid transformation if:
- no side effects, including pattern match errors, can occur
in the let (or x1 <- return ...).
- movement doesnt change the dependencies
- x1 isnt used between the original line and its new position
- there are no new bindings for x2' introduced between the original
line and the new line.
Did I overlook anything? Do any haskell implementations allow rewrites
like these to occur?
Tim Newsham
http://www.thenewsh.com/~newsham/
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