This chapter acts as a reference guide for the particular dialect of
the Scheme programming language used in the Festival Speech Synthesis
systems. The Scheme programming language is a dialect of Lisp
designed to be more consistent. It was chosen for the basic
scripting language in Festival because:
it is a very easy language for machines to parse and interpret, thus
the foot print for the interpreter proper is very small
it offers garbage collection making managing objects safe and easy.
it offers a general consistent datastructure for representing parameters,
rules etc.
it was familiar to the authors
its is suitable for use as an embedded system
Having a scripting language in Festival is actually one of the
fundamental properties that makes Festival a useful system. The fact
that new voices and languages in many cases can be added without
changing the underlying C++ code makes the system mouch more powerful
and accessible than a more monolithic system that requires recompilation
for any parameter changes. As there is sometimes confusion we
should make it clear that Festival contains its own Scheme interpreter
as part of the system. Festival can be view as a Scheme interpreter
that has had basic addition to its function to include modules
that can do speech synthesis, no external Scheme interperter is
required to use Festival.
The actual interpreter used in Festival is based on George Carret's
SIOD, "Scheme in one Defun". But this has been substantially enhanced
from its small elegant beginnings into something that might be better
called "Scheme in one directory". Although there is a standard for
Scheme the version in Festival does not fully follow it, for both good
and bad reasons. Thus finding in order for people to be able to program
in Festival's Scheme we provide this chapter to list the core type,
functions, etc and some examples. We do not pretend to be teaching
programming here but as we know many people who are interested in
building voices are not primarily programmers, some guidance on the
language and its usage will make the simple programming that is
required in building voices, more accessible.
For reference the Scheme Revised Revised Revised report describes the
standard definition [srrrr90]. For a good introduction to
programming in general that happens to use Scheme as its example
language we recommend [abelson85]. Also for those who are
unfamiliar with the use of Lisp-like scripting languages we recommend a
close look as GNU Emacs which uses Lisp as its underlying scripting
language, knowledge of the internals of Emacs did to some extent
influence the scripting language design of Festival.
"Lots of brackets" is what comes to most people's minds when considering
Lisp and its various derivatives such as Scheme. At the start this can
seem daunting and it is true that parenthesis errors can cuase problems.
But with an editor that does proper bracket matching, brackets can
actually be helpful in code structure rather than a hindrance.
The fundamental structure is the s-expression. It consists of an
atom, or a list of s-expressions. This simply defined recursive
structure allows complex structures to easily be specified. For
example
3
(1 2 3)
(a (b c) d)
((a b) (d e))
Unlike other programming languages Scheme's data and code are
in the same format, s-expressions. Thus s-expression
are evaluated, recursively.
- Symbols:
are treated as variables and evaluated return their currently set value.
- Strings and numbers:
evalutate to themselves.
- Lists:
The each member of the list is evaluated and the first item in
the list is treated as a function and applied using the remainer of the
list as arguments to the function.
Thus the s-expression
(+ 1 2)
when evaluated will return
3 as the symbol
+
is bound to a function that adds it arguments.
Variables may be set using the set! function which takes
a variable name and a value as arguments
(set! a 3)
The
set! function is unusual in that it does not evaluate its
first argument. If it did you have to explcitly quote it or set some
other variable to have a value of
a to get the desired effect.
quoting, define
