feat: in app elaborator, beta reduce when substituting arguments #7991
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This PR modifies the app elaborator so that as arguments are elaborated, they are substituted into the remaining function type with beta reduction. This is useful for eliminator-like applications. In an eliminator, if a user provides an explicit motive, it was the case that the expression would be beta reduced, but now it is beta reduced like in the `@[elab_as_elim]` procedure:
```lean4
example (xs : List α) : xs ++ [] = xs :=
List.recOn xs
(motive := fun xs' => xs' ++ [] = xs')
(nil := rfl)
(cons := fun x xs' ih => by rw [List.cons_append, ih])
/- ^
Before:
⊢ ∀ (head : α) (tail : List α),
(fun xs' ↦ xs' ++ [] = xs') tail → (fun xs' ↦ xs' ++ [] = xs') (head :: tail)
After (and before when `motive` is deleted):
⊢ ∀ (head : α) (tail : List α),
tail ++ [] = tail → head :: tail ++ [] = head :: tail
-/
```
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This PR modifies the app elaborator so that as arguments are elaborated, they are substituted into the remaining function type with beta reduction. This is useful for eliminator-like applications. In an eliminator, if a user provides an explicit motive, it used to be that that the expression was substituted without beta reduction, even though the
@[elab_as_elim]procedure would beta reduce the substituted motive. Now a user-provided motive is similarly beta reduced: