Category:TXR

TXR
This programming language may be used to instruct a computer to perform a task.

Official website

See Also:	TXR on the HOPL

Listed below are all of the tasks on Rosetta Code which have been solved using TXR.

TXR is a new language implemented in C, running on POSIX platforms such as Linux, Mac OS X and on Windows via Cygwin as well as more natively thanks to MinGW.

TXR started as a language for "reversing here-documents": evaluating a template of text containing variables, plus useful pattern matching directives, against some body of text and binding pieces of the text which matches variables. The variable bindings were output in POSIX shell variable assignment syntax, allowing for shell code like

eval $(txr <txr-program> <args> ...)

TXR remains close to these roots: its main language is the pattern-based text extraction notation well suited for matching large regions of entire text documents.

About the @ character: this serves as a multi-level escape in TXR. In the fundamental TXR syntax, which is literal text, this character is a signal which indicates that the object which follows is a variable or directive. Then inside a directive, the character indicates that the object which follows is a TXR Lisp expression to be evaluated as such, rather than according to the expression evaluation rules of the pattern language. (And it is possible for TXR Lisp code to give this character additional meanings, since inside TXR Lisp, the notation expands to Lisp syntax: @foo denotes (sys:var foo), and @(foo ...) denotes (sys:expr foo>). In any context which needs to separate meta-variables and meta-expressions from variables and expressions, this may come in handy.)

Thus, the source of a TXR query is literal text except for directives and variables preceded by the @ character.

In this pattern language, computation evolves by textual pattern matching with implicit backtracking. Non-pattern matching activities are embedded into a pattern matching paradigm. For instance, the line

Four score and seven years ago,

is a TXR directive which matches a line of that exact text, or else fails. The following is also a directive:

<lang txr>@(bind foo "abc")</lang>

which succeeds if foo has no prior binding, or already contains "abc", but fails if foo has a binding to something other than "abc".

The success of a directive means that computation proceeds to the next directive (and, if this is a pattern match, the input position advances). Failure means that the enclosing query fails, triggering back-tracking behaviors and possibly failure of the entire query.

A bizarre feature of TXR is that the directives like @(collect) are independent pieces of Lisp. But, they are also de-facto "tokens" in a block-structure language. For instance @(collect) starts a block, which must be terminated by @(end). Inside @(collect) there can be additional syntax, such as @(collect :gap 0 :vars (a b c)).

Extremely Simple Query

Extract first matching line from the password file. Of course, TXR matches are not restricted to single line!

<lang bash>$ txr -c '@(skip) @user:@pw:@uid:@gid:@gecos:@home:@shell' /etc/passwd</lang>

user="root"
pw="x"
uid="0"
gid="0"
gecos="root"
home="/root"
shell="/bin/bash"

Now, pre-bind the user variable first to retrieve some other user. Now @(skip) kicks into action, skipping non-matching material:

<lang bash>$ txr -Duser=sshd -c '@(skip) @user:@pw:@uid:@gid:@gecos:@home:@shell' /etc/passwd</lang>

user="sshd"
pw="x"
uid="114"
gid="65534"
gecos=""
home="/var/run/sshd"
shell="/usr/sbin/nologin"

Simple Query

Re-implement the "free" utility on Linux, showing an overview of memory use:

<lang txr>#!/usr/bin/txr -f @(next "/proc/meminfo") @(define num (n))@\

 @(local numtok)@{numtok /[0-9]+/}@(bind n @(int-str numtok 10))@\

@(end) @(gather) MemTotal: @(num TOTAL) kB MemFree: @(num FREE) kB Buffers: @(num BUFS) kB Cached: @(num CACHED) kB SwapTotal: @(num SWTOT) kB SwapFree: @(num SWFRE) kB @(end) @(bind USED @(- TOTAL FREE 10)) @(bind RUSED @(- USED BUFS CACHED)) @(bind RFREE @(+ FREE BUFS CACHED)) @(bind SWUSE @(- SWTOT SWFRE)) @(output)

              TOTAL         USED         FREE      BUFFERS       CACHED

Mem: @{TOTAL -12} @{USED -12} @{FREE -12} @{BUFS -12} @{CACHED -12} +/- buffers/cache: @{RUSED -12} @{RFREE -12} Swap: @{SWTOT -12} @{SWUSE -12} @{SWFRE -12} @(end)</lang>

Sample run:

$ ./meminfo.txr
               TOTAL         USED         FREE      BUFFERS       CACHED
Mem:          769280       647752       121528       160108       286844
+/- buffers/cache:         200800       568480
Swap:        1048568        18200      1030368

Complex Query

Here is a TXR query for computing the complete dependencies of a C source file (including system and compiler headers) on a typical GNU/Linux system, demonstrating features like parallel clauses, recursion and exception handling.

<lang txr>@(define process_file (dir file already_visited visited_out)) @ (local this_file next_file header_dir next_dir next_dir2 visited_out_next) @ (bind this_file `@dir@file`) @ (none) @ (bind already_visited this_file) @ (end) @ (merge already_visited already_visited this_file) @ (next this_file) @ (collect) @ (cases)

1. include "@*header_dir/@next_file"

@ (bind next_dir `@dir@header_dir/`) @ (or)

1. include "@next_file"

@ (bind next_dir dir) @ (or)

1. @/ */include <@*header_dir/@next_file>

@ (bind next_dir `@sys_includes@header_dir/`) @ (bind next_dir2 `@gcc_includes@header_dir/`) @ (or)

1. @/ */include <@next_file>

@ (bind next_dir sys_includes) @ (bind next_dir2 gcc_includes) @ (or)

1. @/ */include_next <@*header_dir/@next_file>

@ (bind next_dir `@gcc_includes@header_dir/`) @ (or)

1. @/ */include_next <@next_file>

@ (bind next_dir gcc_includes) @ (end) @ (try) @ (process_file next_dir next_file already_visited visited_out_next) @ (merge already_visited already_visited visited_out_next) @ (catch file_error) @ (try) @ (process_file next_dir2 next_file already_visited visited_out_next) @ (merge already_visited already_visited visited_out_next) @ (catch file_error) @ (end) @ (end) @ (end) @ (bind visited_out this_file) @ (try) @ (flatten visited_out_next) @ (merge visited_out visited_out visited_out_next) @ (catch) @ (end) @(end) @(next :args) @(cases) @*dir/@*file.@suffix @ (bind directory `@dir/`) @(or) @*file.@suffix @ (bind directory "") @(end) @(next `!gcc -print-search-dirs`) install: @gcc_install @(bind gcc_includes `@{gcc_install}include/`) @(bind sys_includes "/usr/include/") @(process_file directory `@file.@suffix` nil list_out) @(output) @(rep) @list_out@(first)@file.o:@(end) @(end)</lang>

Sample run:

$ txr dep.txr match.c
match.o: /usr/include/stdio.h /usr/include/features.h /usr/include/sys/cdefs.h /usr/include/bits/wordsize.h /usr/include/gnu/stubs.h /usr/include/gnu/stubs-32.h /usr/lib/gcc/i586-redhat-linux/4.4.1/include/stddef.h /usr/include/bits/types.h /usr/include/libio.h /usr/include/_G_config.h /usr/include/wchar.h /usr/lib/gcc/i586-redhat-linux/4.4.1/include/stdarg.h /usr/include/bits/wchar.h /usr/include/wctype.h /usr/include/endian.h /usr/include/bits/endian.h /usr/include/bits/byteswap.h /usr/include/xlocale.h /usr/include/bits/wchar2.h /usr/include/bits/wchar-ldbl.h /usr/include/gconv.h /usr/include/bits/stdio-lock.h /usr/include/bits/libc-lock.h /usr/include/pthread.h /usr/include/sched.h /usr/include/time.h /usr/include/bits/time.h /usr/include/bits/sched.h /usr/include/signal.h /usr/include/bits/signum.h /usr/include/bits/siginfo.h /usr/include/bits/sigaction.h /usr/include/bits/sigcontext.h /usr/include/asm/sigcontext.h /usr/include/linux/types.h /usr/include/linux/posix_types.h /usr/include/linux/stddef.h /usr/include/asm/posix_types.h /usr/include/asm/types.h /usr/include/asm-generic/int-ll64.h /usr/include/bits/sigstack.h /usr/include/sys/ucontext.h /usr/include/bits/pthreadtypes.h /usr/include/bits/sigthread.h /usr/include/bits/setjmp.h /usr/include/bits/libio-ldbl.h /usr/include/bits/stdio_lim.h /usr/include/bits/sys_errlist.h /usr/include/getopt.h /usr/include/ctype.h /usr/include/bits/stdio.h /usr/include/bits/stdio2.h /usr/include/bits/stdio-ldbl.h /usr/include/stdlib.h /usr/include/bits/waitflags.h /usr/include/bits/waitstatus.h /usr/include/alloca.h /usr/include/bits/stdlib.h /usr/include/bits/stdlib-ldbl.h /usr/include/string.h /usr/include/bits/string.h /usr/include/bits/string2.h /usr/include/bits/string3.h /usr/include/assert.h /usr/include/errno.h /usr/include/bits/errno.h /usr/include/linux/errno.h /usr/include/asm/errno.h /usr/include/asm-generic/errno.h /usr/include/asm-generic/errno-base.h /usr/include/dirent.h /usr/include/bits/dirent.h /usr/include/bits/posix1_lim.h /usr/include/bits/local_lim.h /usr/include/linux/limits.h /usr/include/setjmp.h config.h lib.h gc.h unwind.h regex.h /usr/include/limits.h /usr/lib/gcc/i586-redhat-linux/4.4.1/include/limits.h /usr/lib/gcc/i586-redhat-linux/4.4.1/include/syslimits.h /usr/include/bits/posix2_lim.h /usr/include/bits/xopen_lim.h stream.h parser.h txr.h utf8.h filter.h match.h