A template language for Rust metaprogramming using partial staging.
This crate lets you run code at compile time to produce the tokens other proc_macros will consume. It is also useful for ad-hoc in-place code generation, and for making templates (like Jinja or Mustache) for your code. It uses its own simple inner language based on handlers, variables, and a user-chosen sigil. The stage separation model of Rust does impose some limitations on what this crate can do.
This package was conceived to allow for a specific kind of refactoring in my fantasy cpu emulator needed to make the code fold more and be easier to do experiments with; that project makes use of a fairly complex proc macro to do quick system emulator generation for experimentation with different system designs in a type safe and integrated way, replacing a huge amount of non-type safe and non-integrated ad hoc gnarly perl scripts and build system code generation. But refactoring things in the use site of giant proc macros is not something that Rust had a good story for, so I ended up having to write that story myself, leading to this crate.
And while this can be used for some gnarly stuff like that, it turns out that it can also be used for simple code templating in ways that declarative macros struggle with too.
We don't have true unquote-splicing, as that is more or less impossible in Rust due to the nature of Rust's stage separation.
cargo add do-with-in
In its simplest form, you can wrap the invocation of some other proc_macro with do_with_in! in order to do metaprogramming during the invocation of the thing - that is, in the input itself - using a fairly basic language.
#[macro_use]
extern crate do_with_in;
use do_with_in::*;
// ...
fn main() {
do_with_in!{
sigil: ~ // One of $, %, #, or ~
do // Separates front matter from body
// Define a handler called `header` that wraps its argument
~(mk header <head><title> ~(run ~1) </title></head>)
html!{
<html>
~(header {"My title"})
</html>
}
}
// ...
}There are examples of the use of do_with_in! for use site metaprogramming in the examples/ directory; an example showing off generation of compile time length safe access functions is at examples/mem.rs. The various handlers are documented on the docs dot rs site; the tests at do_with_in_internal_macros/tests/do_with_in_test.rs are a lot of simple cases exercising the functionality of the do_with_in! macro.
This is the proc_macro most users of this crate will use.
There is front matter, which can define the sigil; the default is $ if no sigil is defined. Then do, then after that is where the metaprogramming can happen.
In the metaprogramming section, variables are identifiers with a sigil prepended. You can create and assign to them with let and var handlers. Numbers with a sigil prepended are special variables that can be set inside a handler; you cannot assign to them with let or var. Brackets with a sigil prepended start a handler invocation; the handler invoked will be the first token inside the brackets, which must be an identifier. Everything else passes through the macro unchanged.
If you are using do_with_in! directly, the environment will be prepopulated with a set of default handlers to make it a 'batteries included' experience. See fn.genericDefaultHandlers for the list of documented default handlers.
Create and assign variables, identifiers with a prepended sigil. The difference between the two is that let does not interpolate during either variable definition or use, whereas var interpolates both.
The value assigned to a variable defined with let will remain unchanged before it is used:
%(let bar = {let y = "bar"; })
%bar
// y == "bar"
Whereas a variable set with var can make reference to other metaprogramming variables:
%(let foo = { 5; })
%(var bar = { %foo })
x = %bar
// x == 5
Evaluate block of code in place. Useful for pass-through arguments when building handlers, or to evaluate an unquoted array:
~(let thing = {~(quote ~x + ~y)})
let z = ~(run ~(unquote ~thing));
Similar to the LISP concept, quote renders its arguments inert, and unquote makes active a quoted argument.
This allows constructing and passing arround of an expression without premature evaluation.
~(let thing = {~(quote ~x + ~y)})
~(let
x = {3}
y = {4}
)
let z = ~(run ~(unquote ~thing)); // z == 7
A simple way to define new handlers at the use site. Allows the use of positional numbered parameters e.g. ~1.
// mk defines the handler..
~(mk embolden <b> ~(run ~1) </b>)
// .. which can then be called with arguments
~(embolden {"World"})
Conditional control flow. Test must return either true or false. Only one branch is expanded and executed.
let x = $(if {true} {4} {5}); // x == 4
// Test could be a $logic handler or set variable
let y = $(if {$(logic true & false)} {1} {2}); // y = 2
Concatenates its arguments into a string.
let x = $(concat 1 "abc" 2);
// x == "1abc2"
Provides basic numeric arithmetic operations and logic comparisons respectively.
// Return type must be specified for arithmetic:
let x = $(arithmetic u64 1 + 1 + 1); // x == 3
let y = $(logic false | ($N < $M))
Full documentation can be found on fn.arithmeticHandler and fn.logicHandler.
Redefine which sigil to use for the scope of the handler.
$(let a = {"foo"})
let a = $(withSigil # #(concat #a "bar"));
Markers are a way to "pass" data from one invocation of do_with_in! to another, allowing patterns such as sharing common
handler definitions and variables between do_with_in! blocks.
The marker handler embeds data in one invocation of do_with_in! in a way that can be loaded by later invocations using runMarkers.
$(marker "optional_name" =>
$(let x = { 3 })
$(mk foo
let $1 = $x * $2;))
This can then be invoked in another do_with_in! block (potentially in the same file) like so:
$(runMarkers Base "path" "to" "file.rs" => "optional_name")
$(foo g 2) // $g == 12
Further documentation can be found on fn.markerHandler and fn.runMarkersHandler.
Basic file inclusion. Path is specified by quoted segments; special unquoted identier Base is used for the crate root.
$(import Base "src" "import.$")
A file: hint can be included in the front matter to allow import to use a relative path.
do_with_in!{
file: "src/importable.rs"
sigil: $
do
$(import "import.$")
}
SEE LICENSE IN LICENSE file.