MetaC is a C extension providing the symbolic programming features of
Lisp and the interactive programming environment of scripting
languages such as Python.

The fundamental language features of interest are pattern matching,
backquote and computed macros. Pattern matching is familiar in many
languages.  Although it is not provided by default in Lisp, it is
readily implemented in Lisp using computed macros.  Backquote is a
generalization of Lisp quotation supporting the insertion of computed
values into the quoted expression. Backquote will be familiar to those
versed in writing Lisp computed macros. Pattern matching and backquote
together allow one to express rewriting.  To rewrite an expression
ones used pattern matching to bind variables to parts of the
expression and uses backquote to construct an expression using the
values of the bound variables.  Computed macros allow arbitrary source
language computation to be used in computing the expansion of a macro.
Computed macros should be viewed as compilers.  A computed macro can
do sophisticated type inference or data-flow analysis as part of macro
expansion.

Packages of computed macros are often used as compilers for
sophisticated languages.  However, given that one is writing a
compiler, C is a preferable target language.  For computed macros to
work smoothly it is important that the source language be the same as
the target language.  The target language should be C.

It is not obvious how to implement light weight quotation for C.
Parsing C is complex.  We bypass the syntactic complexity of C by
introducing ``universal algebraic syntax''.  Universal algebraic
syntax is a compromise between the extreme simplicity of Lisp syntax
and the desire for a more user-friendly algebraic syntax with features
like parenthesis-free expressions with standard operator precedence
conventions. While universal algebraic syntax has a variety of
user-friendly features it remains relatively simple --- it is
specified below in eight short bullet points.  The universal algebraic
syntax is universal in three senses.  First, as in Lisp, the syntactic
trees are universal in the sense that they can be assigned any desired
semantics.  Second, any semantics for the input character strings is
preserved in the conversion of strings to trees. In particular, C
semantics can be assigned to a tree by printing the tree as a string
and passing that string to a C compiler.  For this to work the
conversion of a string to an abstract syntax tree (the reader) must
preserve the string --- the printer must invert the reader.  The
reader can then be used for many languages other than C.  A third
sense of universality is that the reader makes very few assumptions
about the string to be read.  Any character string with balanced
parentheses, braces, brackets and quotations can be read to produce an
abstract syntax tree while preserving the information in the string.

See /manual/manual.pdf for more information.