hyung-hwan/hawk

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Hawk

Language
Basic Modules
Embedding Guide

Language

Hawk implements most of the AWK programming language elements with extensions.

Program Structure

A Hawk program is composed of the following elements at the top level.

pattern-action block pair
- BEGIN action block pair
- END action block pair
- action block without a pattern
- pattern without an action block
user-defined function
@global variable declaration
@include directive
@pragma directive

However, none of the above is mandatory. Hawk accepts an empty program.

Pattern-Action Block Pair

A pattern-action pair is composed of a pattern and an action block as shown below:

pattern {
	statement
	statement
	...
}

A pattern can be one of the followings when specified:

expression
first-expression, last-expression
BEGIN
END

An action block is a series of statements enclosed in a curly bracket pair. The BEGIN and END patterns require an action block while normal patterns don't. When no action block is specified for a normal pattern, it is treated as if { print $0; } is specified.

Hawk executes the action block for the BEGIN pattern when it starts executing a program; No start-up action is taken if no BEGIN pattern-action pair is specified. If a normal pattern-action pair and/or the END pattern-action is specified, it reads the standard input stream. For each input line it reads, it checks if a normal pattern expression evaluates to true. For each pattern that evaluates to true, it executes the action block specified for the pattern. When it reaches the end of the input stream, it executes the action block for the END pattern.

Hawk allows zero or more BEGIN patterns. When multiple BEGIN patterns are specified, it executes their action blocks in their appearance order in the program. The same applies to the END patterns and their action blocks. It doesn't read the standard input stream for programs composed of BEGIN blocks only whereas it reads the stream as long as there is an action block for the END pattern or a normal pattern. It evaluates an empty pattern to true; As a result, the action block for an empty pattern is executed for all input lines read.

You can compose a pattern range by putting 2 patterns separated by a comma. The pattern range evaluates to true once the first expression evaluates to true until the last expression evaluates to true.

The following code snippet is a valid Hawk program that prints the string hello, world to the console and exits.

BEGIN {
	print "hello, world";
}

This program prints "hello, world" followed by "hello, all" to the console.

BEGIN {
	print "hello, world";
}
BEGIN {
	print "hello, all";
}

For the following text input,

abcdefgahijklmn
1234567890
opqrstuvwxyzabc
9876543210

this program

BEGIN { mr=0; my_nr=0; }
/abc/ { print "[" $0 "]"; mr++; }
{ my_nr++; }
END { 
	print "total records: " NR; 
	print "total records selfcounted: " my_nr; 
	print "matching records: " mr; 
}

produces the output text like this:

[abcdefgahijklmn]
[opqrstuvwxyzabc]
total records: 4
total records selfcounted: 4
matching records: 2

See the table for the order of execution indicated by the number and the result of pattern evaluation enclosed in parenthesis. The action block is executed if the evaluation result is true.

	START-UP	abcdefgahijklmn	1234567890	opqrstuvwxyzabc	9876543210	SHUTDOWN
`BEGIN { mr = 0; my_nr=0; }`	1(true)
`/abc/ { print "[" $0 "]"; mr++; }`		2(true)	4(false)	6(true)	8(false)
`{ my_nr++; }`		3(true)	5(true)	7(true)	9(true)
`END { print ... }`						10(true)

For the same input, this program shows how to use a ranged pattern.

/abc/,/stu/ { print "[" $0 "]"; }

It produces the output text like this:

[abcdefgahijklmn]
[1234567890]
[opqrstuvwxyzabc]

The regular expression /abc/ matches the first input line and /stu/ matches the third input line. So the range is true between the first input line and the third input line inclusive.

Entry Point

The typical execution begins with the BEGIN block, goes through pattern-action blocks, and eaches the END block. If you like to use a function as an entry point, you may set a function name with @pragma entry.

@pragma entry main

function main ()
{
	print "hello, world";
}

Values

unitialized value
integer
floating-point number
string
byte string
array
function
regular expression

To know the current type of a value, call typename().

function f() { return 10; }
BEGIN { 
	a="hello";
	b=12345;
	print typename(a), typename(b), typename(c), typename(f), typename(1.23), typename(B"world");
}

A regular expression literal is special in that it never appears as an indendent value and still entails a match operation against $0 without an match operator.

BEGIN { $0="ab"; print /ab/, typename(/ab/); }

For this reason, there is no way to get the type name of a regular expressin literal.

Pragmas

A pragma item of the file scope can be placed in any source files. A pragma item of the global scope can appear only once thoughout the all source files.

	Scope	Values	Description
implicit	file	on, off	allow undeclared variables
multilinestr	file	on, off	allow a multiline string literal without continuation
entry	global	function name	change the program entry point
striprecspc	global	on, off
striprecspc	global	on, off	trim leading and trailing spaces when convering a string to a number

Module

Incompatibility with AWK

Parameter passing

In AWK, the caller can pass an uninitialized variable as a function parameter and get a changed value if the callled function sets it to an array.

function q(a) {a[1]=20; a[2]=30;}
BEGIN { q(x); for (i in x) print i, x[i]; }

In Hawk, you can prefix the pramater name with & to indicate call-by-reference for the same effect.

function q(&a) {a[1]=20; a[2]=30;}
BEGIN { q(x); for (i in x) print i, x[i]; }

Alternatively, you may form an array before passing it to a function.

function q(a) {a[1]=20; a[2]=30;}
BEGIN { x[3]=99; q(x); for (i in x) print i, x[i]; }'

Positional variable expression

There are subtle differences when you put an expression for a position variable. In Hawk, most of the ambiguity issues are resolved if you enclose the expression inside parentheses.

	HAWK	AWK
$++$++i	syntax error	OK
$(++$(++i))	OK	syntax error

Basic Modules

sys

ffi

mysql

Embedding Guide

To use hawk in your program, do the followings:

create a hawk instance
parse a source script
create a runtime context
trigger execution on the runtime context
destroy the runtime context
destroy the hawk instance

The following sample illustrates the basic steps hightlighed above.

#include <hawk-std.h>
#include <stdio.h>
#include <string.h>

static const hawk_bch_t* src =
    "BEGIN {"
    "   for (i=2;i<=9;i++)"
    "   {"
    "       for (j=1;j<=9;j++)"
    "           print i \"*\" j \"=\" i * j;"
    "       print \"---------------------\";"
    "   }"
    "}";

int main ()
{
	hawk_t* hawk = HAWK_NULL;
	hawk_rtx_t* rtx = HAWK_NULL;
	hawk_val_t* retv;
	hawk_parsestd_t psin[2];
	int ret;

	hawk = hawk_openstd(0, HAWK_NULL); /* create a hawk instance */
	if (!hawk)  
	{
		fprintf (stderr, "ERROR: cannot open hawk\n");
		ret = -1; goto oops;
	}

	/* set up source script file to read in */
	memset (&psin, 0, HAWK_SIZEOF(psin));
	psin[0].type = HAWK_PARSESTD_BCS;  /* specify the first script path */
	psin[0].u.bcs.ptr = (hawk_bch_t*)src;
	psin[0].u.bcs.len = hawk_count_bcstr(src);
	psin[1].type = HAWK_PARSESTD_NULL; /* indicate the no more script to read */

	ret = hawk_parsestd(hawk, psin, HAWK_NULL); /* parse the script */
	if (ret <= -1)
	{
		hawk_logbfmt (hawk, HAWK_LOG_STDERR, "ERROR(parse): %js\n", hawk_geterrmsg(hawk));
		ret = -1; goto oops;
	}

	/* create a runtime context needed for execution */
	rtx = hawk_rtx_openstd (
		hawk, 
		0,
		HAWK_T("hawk02"),
		HAWK_NULL,  /* stdin */
		HAWK_NULL,  /* stdout */
		HAWK_NULL   /* default cmgr */
	);
	if (!rtx)
	{
		hawk_logbfmt (hawk, HAWK_LOG_STDERR, "ERROR(rtx_open): %js\n", hawk_geterrmsg(hawk));
		ret = -1; goto oops;
	}

	/* execute the BEGIN/pattern-action/END blocks */
	retv = hawk_rtx_loop(rtx);
	if (!retv)
	{
		hawk_logbfmt (hawk, HAWK_LOG_STDERR, "ERROR(rtx_loop): %js\n", hawk_geterrmsg(hawk));
		ret = -1; goto oops;
	}

	/* lowered the reference count of the returned value */
	hawk_rtx_refdownval (rtx, retv);
	ret = 0;

oops:
	if (rtx) hawk_rtx_close (rtx); /* destroy the runtime context */
	if (hawk) hawk_close (hawk); /* destroy the hawk instance */
	return -1;
}

If you prefer C++, you may use the Hawk/HawkStd wrapper classes to simplify the task. The C++ classes are inferior to the C equivalents in that they don't allow creation of multiple runtime contexts over a single hawk instance.

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