User defined pipe and redirection operators... a code tribute to (my annoying cousin) Warren's encouragement... :-)

Note:

1. These "pipes" can be any type (including tagged unions), in essence you have "strong typed" pipes (compile time checked) ...  Perfect for a message passing system.
2. The shell "&" can also be implemented.  Happily the "&" is trivial when implemented with Algol68's parallel clause...  

Hope you enjoy this "space-aged" code (a.k.a. 1960s code)... :-)

 File: Iterator_pipe_operators.a68

MODE
  PAGEIN =         PAGE,
  PAGEAPPEND = REF PAGE,
  PAGEOUT =    REF PAGE;
 
MODE
  MOID = VOID,
  YIELDLINE = PROC(LINE)VOID,
  GENLINE = PROC(YIELDLINE)VOID,
  FILTER = PROC(GENLINE)GENLINE, # the classic shell filter #
  MANYTOONE = PROC([]GENLINE)GENLINE; # eg cat, as in con[cat]enate #
 
PRIO =: = 5, << = 5, >> = 5;
 
OP <  = (FILTER filter, PAGEIN page)GENLINE: filter(READ page),
   <  = (MANYTOONE cmd, PAGEIN page)GENLINE: cmd(READ page),
   << = (FILTER filter, PAGEIN page)GENLINE: filter(READ page),
   >  = (GENLINE gen, PAGEOUT page)VOID: gen(WRITE page),
   >> = (GENLINE gen, PAGEAPPEND page)VOID: gen(APPEND page),
   =: = (GENLINE gen, FILTER filter)GENLINE: filter(gen),
   =: = (GENLINE gen, MANYTOONE cmd)GENLINE: cmd(gen);

File: Iterator_pipe_utilities.a68

• c.f. User defined pipe and redirection operators/ALGOL 68#Iterator pipe utilities

 # Sample ''utilities'' PROCedure declarations #
PROC cat = ([]GENLINE argv)GENLINE:~;
PROC tee = ([]YIELDLINE args filter)FILTER:~;
PROC grep = (STRING pattern, []GENLINE argv)GENLINE:~;
PROC uniq = (GENLINE arg)GENLINE:~;
PROC sort = (GENLINE arg)GENLINE:~;
PROC head = (INT n, []GENLINE args)GENLINE:~;
PROC tail = (INT n, []GENLINE args)GENLINE:~;

File: Iterator_pipe_page.a68

• c.f. User defined pipe and redirection operators/ALGOL 68#Iterator pipe page

# Sample ''pipe I/O'' OPerator declarations #
OP READ = (PAGEIN page)GENLINE:~;
OP WRITE = (PAGEOUT page)YIELDLINE: ~;
OP APPEND = (PAGEAPPEND page)YIELDLINE:~;

File: test_Iterator_pipe_page.a68

#!/usr/local/bin/a68g --script #
# First define what kind of record (aka LINE) we are piping and filtering #
FORMAT line fmt = $xg$;
MODE
  LINE = STRING,
  PAGE = FLEX[0]LINE,
  BOOK = FLEX[0]PAGE;
 
PR READ "Iterator_pipe_page.a68" PR
PR READ "Iterator_pipe_operators.a68" PR
PR READ "Iterator_pipe_utilities.a68" PR
 
PAGE list of computer scientists = (
  "Wil van der Aalst - business process management, process mining, Petri nets",
  "Hal Abelson - intersection of computing and teaching",
  "Serge Abiteboul - database theory",
  "Samson Abramsky - game semantics",
  "Leonard Adleman - RSA, DNA computing",
  "Manindra Agrawal - polynomial-time primality testing",
  "Luis von Ahn - human-based computation",
  "Alfred Aho - compilers book, the 'a' in AWK",
  "Stephen R. Bourne - Bourne shell, portable ALGOL 68C compiler",
  "Kees Koster - ALGOL 68",
  "Lambert Meertens - ALGOL 68, ABC (programming language)",
  "Peter Naur - BNF, ALGOL 60",
  "Guido van Rossum - Python (programming language)",
  "Adriaan van Wijngaarden - Dutch pioneer; ARRA, ALGOL",
  "Dennis E. Wisnosky - Integrated Computer-Aided Manufacturing (ICAM), IDEF",
  "Stephen Wolfram - Mathematica",
  "William Wulf - compilers",
  "Edward Yourdon - Structured Systems Analysis and Design Method",
  "Lotfi Zadeh - fuzzy logic",
  "Arif Zaman - Pseudo-random number generator",
  "Albert Zomaya - Australian pioneer of scheduling in parallel and distributed systems",
  "Konrad Zuse - German pioneer of hardware and software"
);
 
PAGE algol pioneers list, the scientists list;
PAGE aa;
 
# Now do a bit of plumbing: #
cat((
    head(4, ) <  list of computer scientists,
    cat(READ list of computer scientists) =: grep("ALGOL", ) =: tee(WRITE algol pioneers list),
    tail(4, READ list of computer scientists)
  )) =: sort =: uniq =: tee(WRITE the scientists list) =: grep("aa", ) >> aa;
 
# Finally check the result: #
printf((
  $"Pioneer: "$, line fmt, aa, $l$,
  $"Number of Algol pioneers: "g(-0)$, UPB algol pioneers list, $l$,
  $"Number of scientists: "g(-0)$, UPB the scientists list, $l$
))

Output:

Pioneer:  Adriaan van Wijngaarden - Dutch pioneer; ARRA, ALGOL
Number of Algol pioneers: 6
Number of scientists: 15