[Haskell-cafe] List comprehension order of evaluation

[Thomas Hartman]

If I understand list comprehensions correctly, what you wrote is the same 
as

do a <- "ab"; 
      b <- "12"; 
      [a:[b]]

which is the same as

"ab" >== \a -> do b <- "12"; [a:[b]]

which is the same as 

"ab" >>= \a -> "12" >>= \b -> [a:[b]]

which is the same as

concat $ map  (  \a -> "12" >>= \b -> [a:[b]] ) "ab"

.... enough desugaring for now 

Point is, yes it's written in stone.

List comprehensions is just syntactic sugar for monad operations.

Good exercise is to take the above expressions and add parenthesis to make 
it easier to understand order of operations. (Still trips me up often 
enough).

Thomas.





Maurí­cio <briqueabraque at yahoo.com> 
Sent by: haskell-cafe-bounces at haskell.org
10/25/2007 05:59 PM

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haskell-cafe at haskell.org
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Subject
[Haskell-cafe] List comprehension order of evaluation






Hi,

Today, if I write:

[a:[b] | a<-"ab" , b<-"12"]

I get:

["a1","a2","b1","b2"]

Are there any guarantees that I'll never
get ["a1","b1","a2","b2"] instead, i.e.,
that the first list will always be the
last one to be fully transversed? Even
if I use a different compiler or a
future version of Haskell?

Reading how list comprehensions are
translated in the Haskell report it
seems the answer is yes. Is that
written in stone? Can compilers do
it in their own different way?

Thanks,
Maurício

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