Pragmatic directives: Difference between revisions

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Racket eschews pragmas that are specified outside of the language. However, one can view Racket's <tt>#lang</tt> mechanism as a much more powerful tool for achieving similar things. For example, normal code starts with <tt>#lang racket</tt> -- giving you a very Scheme-like language; change it to <tt>#lang typed/racket</tt> and you get a similar language that is statically typed; use <tt>#lang lazy</tt> and you get a Racket-like language that has lazy semantics; use <tt>#lang algol60</tt> and you get something that is very different than Racket. (And of course, you can implement your own language quite easily.)

=={{header|REXX}}==

The REXX language has five pragmatic statements:

::*   NUMERIC DIGITS   {nnn}

::*   NUMERIC FORM     {ENGINEERING | SCIENTIFIC}

::*   NUMERIC FUZZ       {nnn}

::*   OPTIONS               {xxx yyy zzz}

::*   TRACE                   {options}

===numeric digits===

The   '''NUMERIC DIGITS nnn'''   statement is used to specify

to the REXX interpreter how many

(significant) decimal digits

are to be used in calculating and storing numbers.

'''nnn'''   can be an expression that evaluates to a

positive integer.

If   '''nnn'''   is omitted, it defaults to   '''9'''.

If no   '''numeric digits'''   statement is used,   the

default for REXX programs is   '''9'''.

It must be greater than the

(current)   '''NUMERIC FUZZ'''   setting.

===numeric fuzz===

The   '''NUMERIC FUZZ nnn'''   statement is used to

specify to the REXX interpreter how many

decimal digits   (at

full precision)   will be ignored while performing an arithmetic

comparison.

'''nnn'''   can be an expression that evaluates to a

non-negative integer.

If   '''nnn'''   is omitted, it defaults to   '''0'''.

If no   '''numeric fuzz'''   statement is used,   the

default for REXX programs is   '''0'''.

It must be less than the

(current)   '''NUMERIC DIGITS'''   setting.

The result of using a positive value for   '''FUZZ'''   is

that the REXX interpreter (temporarily) reduces

the number of   '''NUMERIC DIGITS'''   by

the   '''FUZZ'''   value before an arithmetic comparison.

This means that arithmetic comparisons are performed with the precision of

'''DIGITS()'''   minus   '''FUZZ()'''   where:

::::*   DIGITS()   is the value of   '''numeric digits'''

::::*   FUZZ()     is the value of   '''numeric fuzz'''

===numeric form===

The   '''NUMERIC FORM'''   statement is used to

cause the REXX interpreter to use a specific form of

exponential format   if   the result of an arithmetic

operation requires the use of exponential notation

with the current value of   '''numeric digits'''.

The specification option after   '''numeric form'''   can be:

::::*   ENGINEERING

::::*   SCIENTIFIC

::::*   (or omitted)

The default is   '''scientific'''.

The option can be in upper/lower/mixed case.

If no   '''numeric form'''   statement is used,   the

default for REXX programs is   '''scientific'''.

===options===

The   '''OPTIONS'''   statement is used to specify

to the REXX interpreter on such matters on how to

process the source (statements), possibly (for instance) whether or

not   ''double byte character strings''  

are present, or possible

cause the REXX interpreter to force compliance to some particular rule or

REXX (program) coding standards.

There can be any number of options listed   (or none).

Each particular REXX interpreters have their

own   '''options''',   so it isn't considered an error if some

option isn't supported (or recognized) by another REXX interpreter.

Some options are global in nature, others can be enabled and disabled.

Some REXX interpreters also have a way to specify certain options via

the   ''command-line''  

(also known as the   ''C.L.'').

===trace===

The   '''TRACE'''   statement is used to cause the REXX

interpreter to turn   ''off''   or ''on''   certain tracing

facilities for the REXX interpreter.

Most tracing options causes some sort of   (tracing of statements

or values of clauses)   to be

emitted to the console (terminal).

The output (tracing information) written to the terminal is usually

quite distinctive and can be easily

recognized.

There are a number of options for

the   '''trace'''   instruction, and they won't be explained here.

=={{header|Tcl}}==