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Merged
Unisay merged 1 commit into from
Jun 25, 2026
Merged

docs: close out the GHC-style Note pass (#44) #130

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7 changes: 7 additions & 0 deletions changelog.d/20260625_203341_unisay_notes_low_items.md
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@@ -0,0 +1,7 @@
### Changed

- Documented the optimizer's `Note [IR is assumed well-typed]`, cited from the
constant-folding and beta-reduction rewrites that rely on it, and made the
conscious calls on the remaining low-priority candidates from #44 (leaving the
already-adequate inline comments in place). This closes out the GHC-style
`Note` documentation pass (#44). Comments only; no change to generated code.
12 changes: 11 additions & 1 deletion lib/Language/PureScript/Backend/IR/Optimizer.hs
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Expand Up @@ -300,13 +300,22 @@ optimizedExpression =
`thenRewrite` removeIfWithEqualBranches
`thenRewrite` inlineLocalBindings

{- Note [IR is assumed well-typed]
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
The optimizer assumes its input IR is well-typed -- it has already passed the
PureScript type checker -- and several rewrites rely on that rather than
re-checking. 'constantFolding' rewrites @Eq True b@ to @b@ on the grounds that
@b@ must then be a 'Bool', and 'betaReduce' substitutes an argument for a
parameter without checking their types match. A rewrite must not introduce an
assumption the type checker would not already guarantee.
-}
constantFolding ∷ RewriteRule Ann
constantFolding =
pure . \case
Eq _ (LiteralBool _ a) (LiteralBool _ b) →
Rewritten Stop $ literalBool $ a == b
Eq _ (LiteralBool _ True) b →
-- 'b' must be of type Bool
-- 'b' must be of type Bool; see Note [IR is assumed well-typed]
Rewritten Stop b
Eq _ (LiteralInt _ a) (LiteralInt _ b) →
Rewritten Stop $ literalBool $ a == b
Expand All @@ -319,6 +328,7 @@ constantFolding =
_ → NoChange

-- (λx. M) N ===> M[x := N]
-- See Note [IR is assumed well-typed]
betaReduce ∷ RewriteRule Ann
betaReduce =
pure . \case
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