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Uniformize destruct and pattern matching logic #1907

Merged
merged 2 commits into from
May 6, 2024
Merged

Uniformize destruct and pattern matching logic #1907

merged 2 commits into from
May 6, 2024

Commits on May 6, 2024

  1. Uniformize destruct and pattern matching logic 

    When patterns were first introduced, only let-destructuring existed, and
match expressions weren't a thing. Rather than to manually generate code
that would check that a pattern does match the destructured value, we
piggy-backed on contracts instead, which was easier and provided better
error messages. Up to now, this was still how destructuring worked: a
contract is elaborated from the pattern and applied to the matched
value, and should report any error if the value doesn't match. Then,
destructuring is desugared to a series of let-bindings that assume that
the destucture value has the right shape. The generated code is thus
quite simple, as it doesn't have any error handling to do.

When pattern matching landed, we kept the old destructuring
infrastructure, because it was there and worked well. However, there is
duplication: the compilation of match expressions does a work quite
similar to destructuring.

What's more, we're at risk of having a diverging semantics. Some
questions aren't trivial to answer for destructuring, in particular with
respect to lazyness: should `let _ = x in null` force `x`? What about
`let 'Foo _ = x in null`, or `let 'Foo 5 = x in null`?

After some discussion, we decided to give destructuring a simple
semantics in term of pattern matching: `let <pat> = <matched> in <body>`
should be strictly equivalent to `<matched> |> match { <pat> => <body>
}`. Indeed, `match` has a clear semantics around forcing values in lazy
languages. This also agrees with intuition, or at least gives a
reasonable semantics for most cases.

This commit implements this change in practice, by getting rid of the
ad-hoc desugaring of destructuring and rewriting let-destructuring to
actual pattern matching instead.
    @yannham
    yannham committed May 6, 2024
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  2. Update core/src/term/pattern/compile.rs 

    Co-authored-by: jneem <joeneeman@gmail.com>
    @yannham @jneem
    yannham and jneem authored May 6, 2024
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