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Fix and improve contract generation optimization for static types #2017
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This commit fixes a number of issues with the optimization of contract generation from a static type annotation. It's been introduced to take advantage of the guarantees of static typing to simplify the generated contract and elide a number of useless checks at run-time. However, this optimization was too agressive as implemented: it just turned any non-function type in a positive position to `Dyn`. Indeed, contracts in negative position must be kept around, because they check what's come from the (potentially untyped) external world: the fact that a function is statically typed doesn't ensure it'll be called with the right arguments. The first issue is that a negative contract can hide inside something else than an arrow type. Typically, take `let functions: Array (Number -> Number) in (std.array.at 0 functions) "a"`. The application is untyped, and wrongly pass a string to a function of the array. However, the current optimization code will turn this contract to just `Dyn` at runtime and get rid of entirely, because there's an `Array _` in positive position. This is only valid if `_` doesn't contain any arrow type, which it does, in this example. Another issue stems from the sealing/unsealing symmetry of polymorphic row contracts. Take `let id : forall r. {foo : Number; r} -> {foo : Number; r} = fun x => x in id {foo = 1, bar = 2}`. The current version would return `{foo = 1}`, with the `bar` field missing. The issue is that the second `{foo : Number; r}` is a record type in positive position, which currently gets entirely elided, giving `forall r. {foo : Number; r} -> Dyn` as an optimized contract. The argument contract seals anything else than `foo` in the record's tail, but there's no dual contract to unseal it anymore, such that it just becomes invisible. This commit implements a more complicated strategy, but which is hopefully correct. Instead of getting rid of type constructor in positive positions, we try to simplify their content as well, and only if the content is simplified down to `Dyn` (or `{; Dyn}` for record rows, etc.), we get rid of the entire constructor. Otherwise, we try to pay only for necessary checks. For example, `{foo : Number, bar : Number -> Number}` now gets simplified to `{bar : Number -> Dyn; Dyn}` instead of just `Dyn` before (which was incorrect). The full simplification rules around polymorphic row contracts are a bit intricate and are properly detailed in the documentation of the corresponding methods.
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Report | Mon, August 5, 2024 at 16:50:18 UTC |
Project | nickel |
Branch | 2017/merge |
Testbed | ubuntu-latest |
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@jneem if you have a chance to take a look - I'd like to include this in the 1.8 release (given the severity of the bug that is fixed) |
jneem
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Aug 5, 2024
yannham
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Aug 5, 2024
yannham
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Aug 5, 2024
yannham
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Aug 5, 2024
Co-authored-by: jneem <joeneeman@gmail.com>
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Fixes #2014.
Motivation
This PR fixes a number of issues with the optimization of contract generation from a static type annotation. This optimization has been introduced to take advantage of the guarantees of static typing to simplify the generated contract and elide a number of useless checks at run-time.
However, the original optimization is too agressive: it turns any non-function type in a positive position to
Dyn
.Contracts in negative position must be kept around, because they check what's come from the (potentially untyped) external world inside the function: the fact that a function is statically typed doesn't ensure it'll be called with the
right arguments. Basic contracts in positive position can be elided though: if a function has been statically checked to be of type
Number -> Number
, we now that it'll always return aNumber
at runtime, and we can simplify the contract toNumber -> Dyn
(which is further specialized to an implementation that doesn't check anything on the codomain). Alas, this isn't true for more complex types in positive positions.Unsoundness
The first issue is that a type constructor in positive position can hide negative contracts inside. Typically, take
let functions: Array (Number -> Number) in (std.array.at 0 functions) "a"
. The application is untyped, and wrongly passes a string to a function of the array. However, the current optimization will turn this contract to justDyn
at runtime and get rid of entirely, because there's anArray _
in positive position, and won't blame at runtime (might get a primop dynamic type error, or nothing, depending what exactly does the function). The elision is only valid if_
doesn't contain any arrow type, which it does, in this example.Another issue stems from the sealing/unsealing symmetry of polymorphic row contracts. Take
let id : forall r. {foo : Number; r} -> {foo : Number; r} = fun x => x in id {foo = 1, bar = 2}
. The current version would return{foo = 1}
, with thebar
field missing. The issue is that the second{foo : Number; r}
is a record type in positive position, which currently gets entirely elided, givingforall r. {foo : Number; r} -> Dyn
as an optimized contract. The argument contract seals anything else thanfoo
in the record's tail, but there's no dual contract to unseal it anymore, such thatbar
stays sealed in the tail forever and thus invisible.This commit implements a more complicated strategy, but which is hopefully correct. Instead of getting rid of type constructor in positive positions, we try to simplify their content as well, and only if the content is simplified down to
Dyn
(or{; Dyn}
for record rows, etc.), we get rid of the entire constructor. Otherwise, we try to pay only for the necessary checks. For example,{foo : Number, bar : Number -> Number}
now gets simplified to{bar : Number -> Dyn; Dyn}
instead of justDyn
before (which was incorrect).The full simplification rules around polymorphic row contracts are a bit intricate and are detailed in the documentation of the corresponding methods.