Formality for Match Ergonomics

This tool formalizes the operation of match ergonomics and implements a number of different proposed rules.

Usage

This tool has three main features:

• For a given (simplified) let statement, show the resulting type of binding under a number of different proposed rulesets.

• For a given set of rules, explain the operation of match ergonomics step by step.

• For a given set of rules, generate a state transition diagram showing how the default binding mode (DBM) is allowed to transition from one state to another based on the pattern and the scrutinee.

To start, type a simple let statement, e.g.:

> let [[x]] = &[&mut [T]];

The tool assumes that all lowercase letters are binding names and all uppercase letters are unit types (that implement Copy). It supports only slices, and it does not support multiple bindings (or commas generally).

When you do this, we'll print out the type of the resulting binding according to a number of different rulesets.

If you want to see this explained, step by step, according to the current stable rules, type:

> explain

Then just press return to proceed through the steps.

If you want to see things explained under a different set of rules, you can set and unset rules.

The orthogonal rules are:

• rule1
• rule2
• rule3
• rule4
• rule4_ext
• rule5
• rule3_ext1
• rule3_lazy
• rule4_early
• spin_rule

And we provide these aliases:

• stable: Unset all rules.
• proposed / rfc: Set rules 1-5 + rule4_ext.
• rpjohnst: Set:
  • rule1
  • rule2
  • rule3_lazy
  • rule4_early
  • spin
• waffle: Set:
  • rule1
  • rule3
  • rule3_ext1
  • rule4_early

So we can write, e.g.:

> set proposed
> unset rule5

...if we want to explore how things work under the proposal without Rule 5.

Once we've set the rules to our liking, we can see a graph of the state transition diagram with:

> graph

This will print a link to a mermaid diagram.

To see your current settings, including all rules applied, type:

> show

Press ctrl-d to exit.

Contributing

This tool is still under heavily development by the author and is not yet seeking outside contributions. Expect force pushes on the master branch.

License

This project is distributed under the terms of both the Apache License (Version 2.0) and the MIT License.

See LICENSE-APACHE and LICENSE-MIT for details.