#3380: [Patch] Support implicit concatenation in list comprehensions
-----------------------------+----------------------------------------------
Reporter: batterseapower | Owner:
Type: feature request | Status: new
Priority: normal | Component: Compiler
Version: 6.10.4 | Severity: normal
Keywords: | Testcase:
Os: Unknown/Multiple | Architecture: Unknown/Multiple
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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.
--
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/3380>
GHC <http://www.haskell.org/ghc/>
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