In your example, yes the two would produce the same result, but the
difference with using "let" inside of a do-block is that you have
access to variables captured inside the block. So for example if you
wanted to get input from the user, capitalize it and append a suffix ,
the following functions are equivalent :
import Data.Char(toUpper)
main = let suffix = "suffix" in
do
x <- getLine
let upperCase = map toUpper x
return (upperCase ++ suffix)
main2 = let suffix = "suffix" in
do
x <- getLine
return (map toUpper x ++ suffix)
Notice that in the "let upperCase ..." line in "main" uses "x" which
was defined within the do-block, you would not have access to "x"
outside the do-block. For reading simplicity when the whole function
is one do-block I usually stick the "let"s inside the block.
-deech
On Tue, Jun 7, 2011 at 11:42 AM, Christopher Howard
On 06/07/2011 04:54 AM, Mats Rauhala wrote:
On 04:30 Tue 07 Jun , Christopher Howard wrote:
In a reply to an earlier question, someone told me that "do" expressions are simply syntactic sugar that expand to expressions with the >> and
= operators. I was trying to understand this (and the Monad class) better.
I see in my own experiments that this...
main = do ln1 <- getLine putStrLn ln1
translates to this:
main = getLine >>= \ln1 -> putStrLn ln1
However, what does this translate to?:
main = do ln1 <- getLine ln2 <- getLine putStrLn (ln1 ++ ln2)
It translates to:
main = getLine >>= \ln1 -> getLine >>= \ln2 -> putStrLn (ln1 ++ ln2)
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Thank you Fischer and Rauhala for your prompt and helpful responses.
One final point of clarification, on a related note: am I correct in reasoning that...
main = do let x = expression /remainder/
is the equivalent of...
main = let x = expression in /expanded remainder/
so that, for example...
main = ln1 <- getline let ln2 = "suffix" putStrLn (ln1 ++ ln2)
would translate to...
main = let ln2 = "suffix" in getLine >>= \ln1 -> putStrLn (ln1 ++ ln2)
This above example works, but I want to be sure I haven't misunderstood any important technical points.
-- frigidcode.com theologia.indicium.us
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