[Patch] Support implicit concatenation in list comprehensions
Using the -XImplicitConcatListComp option turns on implicit concatenation in list comprehensions. What this means is that you can write several things separated by commas in the "body" of a list comprehension, like so
[x, x | x <- [1..10], x `mod` 2 == 0]
This program produces the following output
[2, 2, 4, 4, 6, 6, 8, 8]
The effect is the same as if you had written
concat [[x, x] | x <- [1..10], x `mod` 2 == 0]
but more concise, and more likely to be optimised well by the compiler.
NB: this was originally proposed in the SQL-like list comprehensions paper.