Crate preinterpret

This crate provides the preinterpret! macro, which works as a simple pre-processor to the token stream, and is designed for declarative macro builders.

It is inspired by the quote and paste crates, and built for declarative macro authors to provide:

• Heightened readability - allowing developers to build more maintainable macros.
• Heightened expressivity - mitigating the need to build custom procedural macros.
• Heightened sensibility - helping developers avoid various declarative macro surprises.

It provides two composable features:

• Variable definition with [!set! #variable = ... ] and variable substition with #variable (think quote for declarative macros).
• A toolkit of simple functions operating on token streams, literals and idents, such as [!ident! Hello #world] (think paste but more comprehesive, and still maintained).

Motivation

Heightened readability

The preinterpret syntax is intended to be immediately intuitive even for people not familiar with the crate. And it enables developers to make more readable macros:

• Developers can name clear concepts in their macro output, and re-use them by name, decreasing code duplication.
• Developers can use variables to subdivide logic inside the macro, without having to resort to creating lots of small, functional helper macros.

A simple example follows, but variable substitution becomes even more useful in larger macros with more boilerplate:

macro_rules! impl_marker_traits {
    {
        impl [
            // The marker traits to implement
            $($trait:ident),* $(,)?
        ] for $type_name:ident
        $(
            // Arbitrary (non-const) type generics
            < $( $lt:tt $( : $clt:tt $(+ $dlt:tt )* )? $( = $deflt:tt)? ),+ >
        )?
    } => {preinterpret::preinterpret!{
        [!set! #impl_generics = $(< $( $lt $( : $clt $(+ $dlt )* )? ),+ >)?]
        [!set! #type_generics = $(< $( $lt ),+ >)?]
        [!set! #my_type = $type_name #type_generics]

        $(
            // Output each marker trait for the type
            impl #impl_generics $trait for #my_type {}
        )*
    }}
}

impl_marker_traits! {
    impl [MarkerTrait1, MarkerTrait2] for MyType<T: Clone>
}

Heightened Expressivity

Preinterpret provides a suite of composable functions to convert token streams, literals and idents. The full list is documented in the Details section.

For example:

macro_rules! make_a_struct_and_getters {
    (
        $name:ident { $($field:ident),* $(,)? }
    ) => {preinterpret::preinterpret!{
        // Define a struct with the given fields
        pub struct $name {
            $(
                $field: String,
            )*
        }

        impl $name {
            $(
                // Define get_X for each field X
                pub fn [!ident! get_ $field](&self) -> &str {
                    &self.$field
                }
            )*
        }
    }}
}
make_a_struct_and_getters! {
  MyStruct { hello, world }
}

Variable assignment works intuitively with the * + ? expansion operators, allowing basic procedural logic, such as creation of loop counts and indices before meta-variables are stabilized.

For example:

macro_rules! count_idents {
    {
        $($item: ident),*
    } => {preinterpret::preinterpret!{
        [!set! #current_index = 0usize]
        $(
            [!ignore! $item] // Loop over the items, but don't output them
            [!set! #current_index = #current_index + 1]
        )*
        [!set! #count = #current_index]
        #count
    }}
}
assert_eq!(count_idents!(a, b, c), 3);

To quickly explain how this works, imagine we evaluate count_idents!(a, b, c). As count_idents! is the most outer macro, it runs first, and expands into the following token stream:

let count = preinterpret::preinterpret!{
  [!set! #current_index = 0usize]
  [!ignore! a]
  [!set! #current_index = #current_index + 1]
  [!ignore! = b]
  [!set! #current_index = #current_index + 1]
  [!ignore! = c]
  [!set! #current_index = #current_index + 1]
  [!set! #count = #current_index]
  #count
};

Now the preinterpret! macro runs, resulting in #count equal to the token stream 0usize + 1 + 1 + 1. This will be improved in future releases by adding support for mathematical operations on integer literals.

Heightened sensibility

Using preinterpret partially mitigates some common areas of confusion when writing declarative macros.

Cartesian metavariable expansion errors

Sometimes you wish to output some loop over one meta-variable, whilst inside the loop of a non-parent meta-variable - in other words, you expect to create a cartesian product across these variables. But the macro evaluator only supports zipping of meta-variables of the same length, and gives an unhelpful error message.

The classical wisdom is to output an internal macro_rules! definition to handle the inner output of the cartesian product as per this stack overflow post, but this isn’t very intuitive.

Standard use of preinterpret avoids this problem entirely, as demonstrated by the first readability example. If written out natively without preinterpret, the iteration of the generics in #impl_generics and #my_type wouldn’t be compatible with the iteration over $trait.//!

Eager macro confusion

User-defined macros are not eager - they take a token stream in, and return a token stream; and further macros can then execute in this token stream.

But confusingly, some compiler built-in macros in the standard library (such as format_args!, concat!, concat_idents! and include!) don’t work like this - they actually inspect their arguments, evaluate any macros inside eagerly, before then operating on the outputted tokens.

Don’t get me wrong - it’s useful that you can nest concat! calls and include! calls - but the fact that these macros use the same syntax as “normal” macros but use different resolution behaviour can cause confusion to developers first learning about macros.

Preinterpet commands also typically interpret their arguments eagerly and recursively, but it tries to be less confusing by:

• Having a clear name (Preinterpet) which suggests eager pre-processing.
• Using a different syntax [!command! ...] to macros to avoid confusion.
• Taking on the functionality of the concat! and concat_idents! macros so they don’t have to be used alongside other macros.

Recursive function paradigm shift

To do anything particularly advanced with declarative macros, you end up needing to conjure up various functional macro helpers to partially apply or re-order grammars. This is quite a paradigm-shift from most rust code.

In quite a few cases, preinterpret can allow developers to avoid writing these recursive helper macros entirely.

Limitations with paste support

The widely used paste crate takes the approach of magically hiding the token types from the developer, by attempting to work out whether a pasted value should be an ident, string or literal.

This works 95% of the time, but in other cases such as in attributes, it can cause developer friction. This proved to be one of the motivating use cases for developing preinterpret.

Preinterpret is more explicit about types, and doesn’t have these issues:

macro_rules! impl_new_type {
    {
        $vis:vis $my_type:ident($my_inner_type:ty)
    } => {preinterpret::preinterpret!{
        #[xyz(as_type = [!string! $my_inner_type])]
        $vis struct $my_type($my_inner_type);
    }}
}

Details

Each command except raw resolves in a nested manner as you would expect:

[!set! #foo = fn [!ident! get_ [!snake_case! Hello World]]()]
#foo // "fn get_hello_world()"

Core commands

• [!set! #foo = Hello] followed by [!set! #foo = #bar(World)] sets the variable #foo to the token stream Hello and #bar to the token stream Hello(World), and outputs no tokens. Using #foo or #bar later on will output the current value in the corresponding variable.
• [!raw! abc #abc [!ident! test]] outputs its contents as-is, without any interpretation, giving the token stream abc #abc [!ident! test].
• [!ignore! $foo] ignores all of its content and outputs no tokens. It is useful to make a declarative macro loop over a meta-variable without outputting it into the resulting stream.

Concatenate and convert commands

Each of these commands functions in three steps:

• Apply the interpreter to the token stream, which recursively executes preinterpret commands.
• Convert each token of the resulting stream into a string, and concatenate these together. String and char literals are unquoted, and this process recurses into groups.
• Apply some command-specific conversion.

The grammar value conversion commands are:

• [!string! X Y " " Z (Hello World)] outputs "XY Z(HelloWorld)"
• [!ident! X Y "Z"] outputs the ident XYZ
• [!literal! 31 u 32] outputs the integer literal 31u32
• [!literal! '"' hello '"'] outputs the string literal "hello"

The supported string conversion commands are:

• [!upper_case! foo_bar] outputs "FOO_BAR"
• [!lower_case! FooBar] outputs "foobar"
• [!snake_case! FooBar] and [!lower_snake_case! FooBar] are equivalent and output "foo_bar"
• [!upper_snake_case! FooBar] outputs "FOO_BAR"
• [!camel_case! foo_bar] and [!upper_camel_case! foo_bar] are equivalent and output "FooBar"
• [!lower_camel_case! foo_bar] outputs "fooBar"
• [!capitalize! fooBar] outputs "FooBar"
• [!decapitalize! FooBar] outputs "fooBar"

To create idents from these methods, simply nest them, like so:

[!ident! get_ [!snake_case! $field_name]]

[!NOTE]

These string conversion methods are designed to work intuitively across a relatively wide class of input strings, but treat all characters which are not lowercase or uppercase as word boundaries.

Such characters get dropped in camel case conversions. This could break up grapheme clusters and cause other non-intuitive behaviour. See the tests in string_conversion.rs for more details.

Future plans

Future plans for extensions are discussed in the preinterpret README.

Macros

• preinterpret
  Runs a simple interpeter over the token stream, allowing for variable assignment and substitution, and a toolkit of commands to simplify code generation.