2015-06-09



Following the Tcl topic of yesterday, Marcus wrote me  a  message
with a Tcl quine:  [0]

     join [split a{a} a] {join [split a{a} a] }

To explain it a bit:

Tcl splits the line in three words:
     join
     [split a{a} a]
     {join [split a{a} a] }

`join' is the command

Command substitution  is  performed  on  the  second  word,  thus
``split a{a} a'' is replaced by the three-worded list:
     {"" "{" "}"}

The third word is used literally,  because  it  was  quoted  with
braces.

Therefore the next step is:
     join
     {"" "{" "}"}
     {join [split a{a} a] }

Then, `join' puts the third word between the list elements of the
second word, like this:

     ""
             {join [split a{a} a] }
     "{"
             {join [split a{a} a] }
     "}"

After removing the quotes, the result is what  we  started  from,
hence it's a quine.


btw:  a{a}  in Tcl is the same as  a'a'  in the Bourne shell.


The other quine on the mentioned website is even more impressive:

     join {{} \{ \}} {join {{} \{ \}} }

It uses the same principle, but without the split detour and with
more braces. ;-)

[0]  http://www.nyx.net/~gthompso/self_tcl.txt


http://marmaro.de/lue/        markus schnalke <meillo@marmaro.de>