lolcat is an 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:
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:
% 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 $x}} 1 1 2 3 3 4
To understand how it works, remember that concat concatenates lists, and concat {*}$ls receives the elements of $ls as arguments. Thus, concat {*}$ls will flatten (one level of) a list:
% 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
PYK 2016-01-09 cautions: lolcat requires that the result of the lmap body be a list, not a simple value. Otherwise, data might get munged.
-- Please provide an example.
PYK 2016-04-19: join is a little more concise and efficient, and the 1 in uplevel isn't strictly necessary since lmap can't be interpreted as a level:
proc lolcat args { join [uplevel lmap $args] }
-- Timing trials indicate that the join version is some 8% slower. YMMV.
One neat extension on lolcat is dictify:
proc dictify {cmdPrefix ls} { lolcat x $ls { list $x [uplevel 1 $cmdPrefix [list $x]] } }
This allows you to make a dictionary whose keys are a list, and values are the result of evaluating a command on each element.
% dictify {expr 2**} {1 2 3 4 5} 1 2 2 4 3 8 4 16 5 32
Or more elaborately:
% 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, and I only stuck with for lack of a better alternative. It took the toad's genius to find this proc's correct name, which just goes to show: give a dog a bad name, and it will stick ... but a cat can change its stripes!
AMG: [lolcat] resembles [lcomp]:
% lolcat {a b} {1 2 3 4} {list $b $a} % lolcat a {1 3} b {2 4} {list $b $a} % lcomp {$b} {$a} for {a b} in {1 2 3 4} % lcomp {$b} {$a} for a in {1 3} and b in {2 4}
All return 2 1 4 3. The differences are:
[lolcat] | [lcomp] |
---|---|
Result generator is last argument | Result generator is first argument(s) |
Result generator is one argument | Result generator is one or more argument(s) |
Result generator is Tcl script | Result generator is expr expressions |
No noise words | Supports numerous tokens such as for |
While I'm on the subject, Brush offers (will offer) capability very similar to [lcomp], though the language-level syntax is different:
% collect b a for (&a &b) in (1 2 3 4) % collect b a for &a in (1 3) and &b in (2 4)