[Haskell-cafe] List comprehension order of evaluation

Thomas Hartman thomas.hartman at db.com
Thu Oct 25 18:15:15 EDT 2007

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

do a <- "ab"; 
      b <- "12"; 

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 


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

haskell-cafe at haskell.org

[Haskell-cafe] List comprehension order of evaluation


Today, if I write:

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

I get:


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?


Haskell-Cafe mailing list
Haskell-Cafe at haskell.org


This e-mail may contain confidential and/or privileged information. If you 
are not the intended recipient (or have received this e-mail in error) 
please notify the sender immediately and destroy this e-mail. Any 
unauthorized copying, disclosure or distribution of the material in this 
e-mail is strictly forbidden.
-------------- next part --------------
An HTML attachment was scrubbed...
URL: http://www.haskell.org/pipermail/haskell-cafe/attachments/20071025/9e40d429/attachment-0001.htm

More information about the Haskell-Cafe mailing list