'''lolcat''' is an [Additional list functions%|%additional list function] which [aspect] uses almost every day.  Once you come to love it, you will too!

It's a portmanteau of [lmap], [{*}] and [concat]:

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proc lolcat args {
    concat {*}[uplevel 1 lmap $args]
}
======

It's useful in cases where you want to [lmap], but the body might need to yield multiple results for a single iteration.  Here's a simple example:

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% lolcat {x y} {1 2 3 4} {list $y $x}
2 1 4 3
% lolcat x {1 2 3 4} {if {$x%2} {list $x $x} else {list $x}}
1 1 2 3 3 4
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To understand how it works, remember that [concat] concatenates lists, and `concat {*}$ls` receives the elements of `$ls` as arguments.  Thus, `concat {*}$ls` will flatten (one level of) a list:

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% concat {*}{{1 2} {3 4 5} {6 {7 8}} 9}
1 2 3 4 5 6 {7 8} 9
# equivalently:
% concat {1 2} {3 4 5} {6 {7 8}} 9
1 2 3 4 5 6 {7 8} 9
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One neat extension on lolcat is '''dictify''':

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proc dictify {cmdPrefix ls} {
    lconcat x $ls {
        list $x [uplevel 1 $cmdPrefix [list $x]]
    }
}
======

This allows you to make a [dict]ionary whose keys are a list, and values are the result of evaluating a command on each element.

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% dictify {expr 2**} {1 2 3 4 5}
1 2 2 4 3 8 4 16 5 32
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Or more elaborately:

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% proc pdict {d} {array set {} $d; parray {}}
% pdict [dictify {tcl::pkgconfig get} [tcl::pkgconfig list]]
debug              = 0
threaded           = 1
profiled           = 0
64bit              = 0
optimized          = 1
mem_debug          = 0
compile_debug      = 0
compile_stats      = 0
libdir,runtime     = /home/tcl/lib
bindir,runtime     = /home/tcl/bin
scriptdir,runtime  = /home/tcl/lib/tcl8.6
includedir,runtime = /home/tcl/include
docdir,runtime     = /home/tcl/man
libdir,install     = /home/tcl/lib
bindir,install     = /home/tcl/bin
scriptdir,install  = /home/tcl/lib/tcl8.6
includedir,install = /home/tcl/include
docdir,install     = /home/tcl/man
======

The name comes courtesy [hypnotoad] - previously I called this procedure '''lconcat''', which is obviously a terrible name.  Perhaps it should really be '''lolmap''', but that's not nearly as cute.