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chore: cleanup Vec.lookup #566

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Sep 20, 2025
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chore: cleanup Vec.lookup #566

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26 changes: 9 additions & 17 deletions src/Data/Vec/Base.lagda.md
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Original file line number Diff line number Diff line change
Expand Up @@ -4,7 +4,6 @@ open import 1Lab.Path
open import 1Lab.Type

open import Data.Product.NAry
open import Data.List.Base hiding (lookup ; tabulate ; _++_)
open import Data.Fin.Base
open import Data.Nat.Base
```
Expand Down Expand Up @@ -42,10 +41,16 @@ private variable
A B C : Type ℓ
n k : Nat

head : Vec A (suc n) → A
head (x ∷ xs) = x

tail : Vec A (suc n) → Vec A n
tail (x ∷ xs) = xs

lookup : Vec A n → Fin n → A
lookup (x ∷ xs) n with fin-view n
... | zero = x
... | suc i = lookup xs i
lookup xs n with fin-view n
... | zero = head xs
... | suc i = lookup (tail xs) i
```

<!--
Expand All @@ -61,19 +66,6 @@ Vec-cast {A = A} {x = x} {y = y} p xs =
head ∷ cast-tail len (suc-inj 1+n=len)
})
xs y p

-- Will always compute:
lookup-safe : Vec A n → Fin n → A
lookup-safe {A = A} xs n =
Fin-elim (λ {n} _ → Vec A n → A)
(λ {k} xs → Vec-elim (λ {k'} _ → suc k ≡ k' → A)
(λ p → absurd (suc≠zero p))
(λ x _ _ _ → x) xs refl)
(λ {i} j cont vec →
Vec-elim (λ {k'} xs → suc i ≡ k' → A)
(λ p → absurd (suc≠zero p))
(λ {n} head tail _ si=sn → cont (Vec-cast (suc-inj (sym si=sn)) tail)) vec refl)
n xs
```
-->

Expand Down
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