For a collection of useful macros, see the associated Magma project
Usage
Macros are written directly in the source, and the cmc program is used to
process a file with macros to a macroexpanded file.
cmc code_with_macros.c -o macroexpanded_code.c
Installing
If you're running Arch or a similarly bleeding-edge distro, just install sbcl
from Pacman and skip to step 5. Otherwise, you need to manually download the
latest SBCL[1].
Unpack it, for example, through bzip2 -cd sbcl-1.1.17-x86-linux-binary.tar.bz2 | tar xvf -
Install git, curl and flex through your favorite package manager.
Build SBCL: cd <sbcl dir>; sudo sh install.sh
Build cmacro: make, sudo make install
[1]: Buildapp doesn't work on older
versions of SBCL, and it is required to build the executable.
What?
A macro is a function that operates on your code's abstract syntax tree rather
than values. Macros in cmacro have nothing to do with the C preprocessor except
they happen at compile time, and have no knowledge of run-time values.
In cmacro, a macro maps patterns in the code to templates. A macro may have
multiple cases, each matching multiple patterns, but each producing code through
the same template.
Macros are not primarily about safety and performance: They are about the
programmer. Macros give you automation, plain and simple. They allow you to
abstract away and remove repetition in places where a functional or
object-oriented approach can't. For example, Common Lisp's
WITH-OPEN-FILE macro helps with the
common pattern of 'acquire a resource, apply something to it, and close
it'. While this can be done in languages that support (And have simple syntax
for) anonymous functions, macros help reduce this syntactic overhead.
cmacro has a very lenient notion of C syntax, which means you can write macros
to implement DSLs with any syntax you like. You could implement Lisp-like prefix
notation, or a DSL for routing URLs, or the decorator pattern, for example.
For a very simple example, this macro matches anything of the form `unless
`, where `` is any arbitrary expression, and performs a simple
transformation:
```c
macro unless {
case {
match {
$(cond)
}
template {
if(!$(cond))
}
}
}
```
With this definition, code like `unless(buffer.empty)` becomes `if(!(buffer.empty))`.
A more complicated macro can match multiple patterns, like the `route` macro
which implements a DSL for defining routes in a hypothetical C web framework.
```c
macro route {
/* Route all requests to 'url' to 'route'. Optionally discriminate by HTTP
method (GET by default). */
case {
match {
$(url) => $(route)
}
template {
register_route($(url), $(route), HTTP_GET);
}
}
case {
match {
$(url) [$(method)] => $(route)
}
template {
register_route($(url), $(route), $(method));
}
}
}
// Usage with the lambda macro (See below)
route "/profile/" =>
lambda(Req* request) -> Resp { return Authenticate(request.user); }
```
# Why?
Because a language without macros is a tool: You write applications with it. A
language with macros is building material: You shape it and grow it *into* your
application.
There is a sweet spot between low-level performance and control and high-level
metaprogramming that is not yet occupied by any language: Metaprogramming, being
an inherently compile-time thing, can be done in the absence of automatic memory
management or dynamic typing. [Rust](http://www.rust-lang.org/) seems to want to
fill this spot, and I also approached this problem with
[Corvus](https://github.com/eudoxia0/corvus), but I feel this approach of adding
metaprogramming to C - A simple language, with a long history, that runs truly
everywhere - can become useful.
# Examples
## `lambda`
```c
macro lambda {
case {
match {
$(args) -> $(ret) $(body)
}
template {
$(@getsym lambda 0)
}
toplevel {
$(ret) $(@gensym lambda) $(args) $(body)
}
}
}
```
Usage:
```c
/* Input */
fn = lambda (int x, int y) -> int { return x + y; };
/* After macroexpansion */
int cmacro_lambda_0(int x, int y) { return x + y; }
fn = cmacro_lambda_0;
```
A more complicated example, using the `qsort` function:
```c
int main() {
int array[] = {423, 61, 957, 133, 969,
829, 821, 390, 704, 596};
qsort(array, 10, sizeof(int),
lambda (const void* a, const void* b) -> int
{ return *(int*)a - *(int*)b; });
for(size_t i = 0; i < 10; i++){
printf("%i ", array[i]);
}
return 0;
}
```
## Anaphoric `if`
This stores the result of the condition in the variable `it`. See
[Anaphora](http://common-lisp.net/project/anaphora/) for a collection of similar
anaphoric macros.
```c
macro aif {
case {
match {
$(cond)
}
template {
typeof($(cond)) it = $(cond);
if(it)
}
}
}
```
Usage:
```c
/* Input*/
aif(get_buffer(a,b,c)) {
write_string(it, text);
}
/* After macroexpansion */
typeof(get_buffer(a,b,c)) it = get_buffer(a,b,c);
if(it) {
write_string(it, text);
}
```
## `forEach`
```c
macro forEach {
case {
match {
($(item), $(collection)) $(body)
}
template {
{
size_t index;
typeof($(collection)[0]) $(item);
for(index = 0, item = nth($(collection), 0);
index < length($(collection));
index++)
$(body)
}
}
}
}
```
# Variables
The syntax for variables is just a name followed by an optional,
space-separated list of *qualifiers*, enclosed in the `$()` operator, eg:
`$(var)`, `$(body ident)`, `$(arg const)`.
## Qualifiers
- None: The variable matches any expression.
- `rest`: Match multiple expressions (Like C's `...`).
- `ident`: Matches Identifiers.
- `int`: Integers.
- `float`: Floats.
- `num`: Integers and floats.
- `string`: String literals.
- `const`: The equivalent of `(or int float string)`.
- `op`: Operators.
- `list`, `array`, `block`: Matches expressions of the form `(...)`, `[...]`,
`{...}`.
# Template operations
These use regular variable syntax but the text starts with a '@'.
- `gensym
