Add a simple exhaustiveness checker 🧐

[jmackie]

More or less a verbatim Rust port of this blog post.

Raises an error for missing patterns, and raises warnings for redundant patterns.

Note that currently the Ditto pattern syntax consists only of constructors and variables:

ditto/crates/ditto-ast/src/expression.rs

Lines 347 to 382 in 2304f8b

	/// A pattern to be matched.
	#[derive(Debug, Clone, Serialize, Deserialize)]
	pub enum Pattern {
	/// A local constructor pattern.
	LocalConstructor {
	/// The source span for this pattern.
	span: Span,
	/// `Just`
	constructor: ProperName,
	/// Pattern arguments to the constructor.
	arguments: Vec<Self>,
	},
	/// An imported constructor pattern.
	ImportedConstructor {
	/// The source span for this pattern.
	span: Span,
	/// `Maybe.Just`
	constructor: FullyQualifiedProperName,
	/// Pattern arguments to the constructor.
	arguments: Vec<Self>,
	},
	/// A variable binding pattern.
	Variable {
	/// The source span for this pattern.
	span: Span,
	/// Name to bind.
	name: Name,
	},
	/// An unused pattern.
	Unused {
	/// The source span for this pattern.
	span: Span,
	/// The unused name.
	unused_name: UnusedName,
	},
	}

But this logic will get more elaborate once we add more pattern variants (e.g. #16).

[jmackie]

Note (to self): got stuck on this bit for a while as the typeclass-y Haskell logic wasn't that obvious...

mconcat <$> (sequence (zipWith hasSubst pats1 pats2))

[codecov]

Codecov Report

Merging #47 (0ece521) into main (2304f8b) will increase coverage by 0.82%.
The diff coverage is 97.20%.

@@            Coverage Diff             @@
##             main      #47      +/-   ##
==========================================
+ Coverage   76.88%   77.70%   +0.82%     
==========================================
  Files         109      111       +2     
  Lines        6948     7195     +247     
==========================================
+ Hits         5342     5591     +249     
+ Misses       1606     1604       -2     

Impacted Files	Coverage Δ
crates/ditto-ast/src/kind.rs	100.00% <ø> (ø)
crates/ditto-ast/src/name.rs	77.77% <ø> (ø)
crates/ditto-ast/src/type.rs	82.55% <ø> (ø)
...ates/ditto-checker/src/typechecker/coverage/mod.rs	96.37% <96.37%> (ø)
...cker/src/module/type_declarations/tests/acyclic.rs	100.00% <100.00%> (ø)
crates/ditto-checker/src/result/type_error.rs	98.92% <100.00%> (+0.03%)	⬆️
crates/ditto-checker/src/result/warnings.rs	100.00% <100.00%> (ø)
...es/ditto-checker/src/typechecker/coverage/tests.rs	100.00% <100.00%> (ø)
crates/ditto-checker/src/typechecker/env.rs	97.87% <100.00%> (ø)
crates/ditto-checker/src/typechecker/mod.rs	96.40% <100.00%> (+0.14%)	⬆️
... and 4 more

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[jmackie]

I'm tempted to surround the rendered pattern in backticks to make it clearer that it's code (rather than prose) but that might be too visually noisy?