I'm reading through the Gentle Introduction and am wondering why in a list comprehension with two generators, the rightmost generator is iterated "the fastest" (i.e. compiles to the innermost loop, I guess). Observe the following GHCi output:
*Main> concat [[(x,y) | x <- [0..2]] | y <- [0..2]]
[(0,0),(1,0),(2,0),(0,1),(1,1),(2,1),(0,2),(1,2),(2,2)]
*Main> [(x,y) | x <- [0..2], y <- [0..2]]
[(0,0),(0,1),(0,2),(1,0),(1,1),(1,2),(2,0),(2,1),(2,2)]
If the leftmost generator were iterated fastest, the above two expressions would have the same value, which I think makes choosing this convention more natural somehow.
So does anyone know why the opposite convention was chosen? I notice Python has the same convention as Haskell (maybe even borrowed it from Haskell?), and in Python world the word seems to be that the ordering was chosen "because that's the order in which you'd write a for loop", but I gather that thinking in terms of for loops is not exactly what most Haskell programmers do...
Thoughts?
From my comment on Louis Wasserman's answer below:
I guess here the order corresponding to an imperative-style explication of the comprehension was considered more natural than having it correspond with nesting the list. So in essence the Haskell explanation for this is the same as the Python explanation I linked in the question, after all, it seems.
So that things scope in a sane way.
[(x, y) | x <- [1..10], y <- [1..x]]
makes sense -- x is in scope for the comprehension on y -- but
[(x, y) | y <- [1..x], x <- [1..10]]
makes somewhat less sense.
Additionally, this way it's consistent with the do monad syntax:
do x <- [1..10]
y <- [1..x]
return (x, y)