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62 lines
2.3 KiB
Agda
62 lines
2.3 KiB
Agda
{-# OPTIONS --rewriting #-}
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open import Luau.Type using (Type; Scalar; nil; number; string; boolean; never; unknown; _⇒_; _∪_; _∩_)
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open import Properties.Equality using (_≢_)
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module Luau.Subtyping where
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-- An implementation of semantic subtyping
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-- We think of types as languages of trees
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data Tree : Set where
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scalar : ∀ {T} → Scalar T → Tree
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function : Tree
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function-ok : Tree → Tree
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function-err : Tree → Tree
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data Language : Type → Tree → Set
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data ¬Language : Type → Tree → Set
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data Language where
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scalar : ∀ {T} → (s : Scalar T) → Language T (scalar s)
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function : ∀ {T U} → Language (T ⇒ U) function
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function-ok : ∀ {T U u} → (Language U u) → Language (T ⇒ U) (function-ok u)
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function-err : ∀ {T U t} → (¬Language T t) → Language (T ⇒ U) (function-err t)
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scalar-function-err : ∀ {S t} → (Scalar S) → Language S (function-err t)
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left : ∀ {T U t} → Language T t → Language (T ∪ U) t
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right : ∀ {T U u} → Language U u → Language (T ∪ U) u
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_,_ : ∀ {T U t} → Language T t → Language U t → Language (T ∩ U) t
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unknown : ∀ {t} → Language unknown t
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data ¬Language where
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scalar-scalar : ∀ {S T} → (s : Scalar S) → (Scalar T) → (S ≢ T) → ¬Language T (scalar s)
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scalar-function : ∀ {S} → (Scalar S) → ¬Language S function
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scalar-function-ok : ∀ {S u} → (Scalar S) → ¬Language S (function-ok u)
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function-scalar : ∀ {S T U} (s : Scalar S) → ¬Language (T ⇒ U) (scalar s)
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function-ok : ∀ {T U u} → (¬Language U u) → ¬Language (T ⇒ U) (function-ok u)
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function-err : ∀ {T U t} → (Language T t) → ¬Language (T ⇒ U) (function-err t)
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_,_ : ∀ {T U t} → ¬Language T t → ¬Language U t → ¬Language (T ∪ U) t
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left : ∀ {T U t} → ¬Language T t → ¬Language (T ∩ U) t
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right : ∀ {T U u} → ¬Language U u → ¬Language (T ∩ U) u
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never : ∀ {t} → ¬Language never t
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-- Subtyping as language inclusion
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_<:_ : Type → Type → Set
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(T <: U) = ∀ t → (Language T t) → (Language U t)
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-- For warnings, we are interested in failures of subtyping,
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-- which is whrn there is a tree in T's language that isn't in U's.
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data _≮:_ (T U : Type) : Set where
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witness : ∀ t →
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Language T t →
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¬Language U t →
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-----------------
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T ≮: U
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