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dry — Don't Repeat Yourself

Latest version Documentation License: MIT/Apache 2.0

Rust macros for idiomatic deduplication of code. Use whenever macro_rules! are too powerful and clunky.

[dependencies]
dry = "0.1.1"

Usage

macro_for!

You know the trusty for loop:

for number in [1, 2, 3, 4, 5] {
  println!("{}", number);
}

Use macro_for! to iterate over tokens at compile time:

macro_for!($Struct in [A, B, C, D, E] {
  struct $Struct {
    many_fields: bool,
    so_many_fields: bool,
    impossibly_many_fields: bool,
  }
});

Compared to using macro_rules!:

macro_rules! my_struct {
  ($Struct:ident) => {
    struct $Struct {
      many_fields: bool,
      so_many_fields: bool,
      impossibly_many_fields: bool,
    }
  };
}
my_struct!(A);
my_struct!(B);
my_struct!(C);
my_struct!(D);
my_struct!(E);

See the examples for more details.

macro_wrap!

Allows you to use the other macros in this crate in places where macro invocations are illegal (e.g. struct fields, enum cases, match arms).

Wrap the closest syntax tree ancestor that is in a macro invocation position and you're good to go:

macro_wrap!(match x {
  // ↓ can't usually call macros here, but `macro_wrap!` makes it work
  macro_for!($Variant in [A, B, C, D, E] {
    Enum::$Variant => 1,
  })
})

Note that because macro_wrap! calls macro_for! directly in order to make this work, you don't need to use dry::macro_for if you're not using it anywhere else.

Features

The nightly feature (disabled by default) enables functionality that uses the unstable proc_macro_span rustc feature. It enables better syntax checking (disallows spaces between the "$" and the substitution variable names) and emits more source code hints on errors (though quick-fixes for macros aren't available even on nightly yet).

If you're running Rust nightly, you can enable it:

[dependencies]
dry = { version = "0.1.1", features = ["nightly"] }

About This Crate

Dependencies

The only dependency is proc-macro-error, for those sweet, sweet, friendly error messages across Rust versions. In turn, it depends on quote and proc-macro2. However, we don't depend on syn at all so dry should be really light on compile times.

Caution

You should try to use an abstraction like looping, traits, or generics whenever possible and practical. But when it's not, dry makes it as painless and pleasant as possible to avoid repeating yourself.

Prior Art

For Each Loops

  • duplicate: The most popular by far and works more or less like a regular for loop with tuple destructuring, except for the very foreign syntax. It offers an attribute syntax which avoids some nesting, but at the cost of clarity, in my opinion. The function-like syntax can be used wherever the attribute sytnax is valid, plus "$"-prefixed identifiers are invalid Rust and therefore not possible to implement in an attribute syntax.
  • akin: Overloads the let syntax with an implicit for comprehension. Avoids nesting for large numbers of substitution variables but feels too magical to feel at home in most Rust codebases, in my opinion.
  • ct-for: Almost there! However it uses in ... do syntax instead of the more familiar in ... {} syntax Rustaceans are used to. This also makes it more difficult for editors to correctly indent the loop body. Finally, the ct in ct_for is not very descriptive.

All of them use bare identifiers instead of "$"-prefixed identifiers like dry and macro_rules! do, which make it clear when macros are being used versus standard language features within the loop body. None of them support replicating struct fields, enum cases, or match arms as far as I'm aware.

For Each Loops Over Macros

N Repetitions

  • repeated: Not quite the same thing, but alerted the author to the macro invocation position (e.g. match arms, etc.) problem and one way to solve it. In fact, a match with many arms for an enum in an external crate without generic traits is what spurred initial development of dry!
  • seq_macro: Inspired the syntax used in macro_for!. Great if you want to idiomatically iterate over a range of numeric or character values instead of a list of tokens at compile time.

License

dry is licensed under the MIT License and the Apache 2.0 License, at your option.

Unless you explicitly state otherwise, any contribution intentionally submitted for inclusion in the work by you, as defined in the Apache-2.0 license, shall be dual licensed as above, without any additional terms or conditions.

Contributing

Please open a pull request if you'd like to see anything fixed or added, or create an issue if you see a problem or missing feature and you're not sure how or don't have the time to fix it.

A great place to start is the roadmap below. The first few ones are small and well-defined enough that they'd make a great first pull request. We'd love to have you on board!

Roadmap to 1.0

  • Idiomatic for-like syntax.
  • Helpful compiler error messages and hints, modelled after rustc's errors for the equivalent runtime constructions.
  • Wrapper for uses where macro invocations are illegal (e.g. struct fields, enum cases, match arms): macro_wrap.
  • Fix bug where adding stuff after the last } is ignored. Should be an error instead.
  • Ignore trailing comma (don't generate an additional empty substitution). Empty substitution can be generated with ().
  • Make it an error to have two consecutive commas. Instead, require a () in between for an empty substitution.
  • Better documentation
  • Testing
  • Support multiple substitution variables using a tuple-destructuring-like syntax
  • Support commas in substitutions by wrapping in parentheses (and support parentheses by doubling them)
  • Figure out minimum Rust version
  • Nesting with scoped substitution variables (currently substitution variables are expanded outside-in, not inside-out like you would expect in a regular for loop)
  • macro_let macro for idiomatic substitutions (replaces macro_rules! without syntax arguments)
  • Investigate joining substitutions with syntax elements in the loop body? Like identifiers ($variable~_suffix), or operators (variable $op~= change). This is meant to be a straightforward replacement for macro_rules! in simple cases, though. How does it solve this problem? See paste crate.
  • Can macro_wrap expand macros outside of this crate, too? Probably not, but let's investigate. Maybe we can let other macro crates plug into it if we can't do it automatically.
  • How to deal with substitutions with repeated items of groups? duplicate solves this with what they call parametrized substitution