4.3 oo.scm

Sometimes it is useful to extend a Festival function in some way. Standard Festival functions don't provide easy to use means for it. This module tries to fill the gap.

The following macro allows you to wrap a defined function:

define-wrapper (function arg ...) wrapper-name . body)

Wrap function with arguments arg ... by the code body. Given function arguments must match the arguments of the wrapped function. wrapper-name is a symbol uniquely identifying the wrapper, it allows redefinition of the wrapper. One function can be wrapped by any number of wrappers. None of the define-wrapper arguments is evaluated.

Within body, a function named next-func is automatically defined. It returns the next wrapper or the original function. Please note next-func is a function, not a variable, so its typical invocation looks as follows: ((next-func) arg ...).

oo-ensure-function-wrapped function-name

If a wrapped function gets redefined, its wrapper is lost. If you want to ensure the function is still wrapped before its use, you may call this function, with its symbol name as the argument.

oo-unwrapped function-name

Return the original definition of a wrapped function.

Example use:

festival> (define (foo x) (+ x 42))
#<CLOSURE (x) (+ x 42)>
festival> (foo 1)
43
festival> (define-wrapper (foo x) my-foo-wrapper (print "Foo called.") ((next-func) x))
nil
festival> (foo 1)
"Foo called."
43

You can also wrap parameters, set by Param.set:

Param.wrap name wrapper-name . body

Wrap access to parameter name by code body. If the given parameter is accessed, its wrapper is invoked instead of just returning the parameter value. wrapper-name is the same as in define-wrapper.

Macro next-value is automatically defined within body. It returns the parameter value, either plain or modified by another wrapper.

Example use of parameter wrapping:

festival> (Param.set 'foo 42)
#<feats 0x8169950>
festival> (Param.wrap foo foo-w (+ (next-value) 1))
nil
festival> (Param.get 'foo)
43

And finally, the glet* macro allows you to dynamically bind a global variable value:

glet* bindings . body

Similar to let* (see section util.scm) except that the variables in bindings are bound dynamically instead of lexically. All variables in bindings must be global variables.

This document was generated by Milan Zamazal on August, 11 2009 using texi2html 1.78.