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feat: add De Bruijn indexed reduction engine #33

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mvhutz wants to merge 2 commits from feat/debruijn-engine into main
pull from: feat/debruijn-engine
merge into: mvhutz:main
Conversation 0 Commits 2 Files Changed 12 +5 -11
1 changed files with 5 additions and 11 deletions
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Showing only changes of commit 21ae2ca91c - Show all commits

16
pkg/debruijn/reducer.go
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@@ -40,16 +40,6 @@ func isViable(e *Expression) (*Abstraction, Expression, bool) {
}
}
// betaReduce performs a single beta reduction step.
// Given (\. body) arg, it substitutes arg for index 0 in body,
// then shifts the result down to account for the removed abstraction.
func betaReduce(fn *Abstraction, arg Expression) Expression {
// Substitute arg for variable 0 in the body.
substituted := Substitute(fn.body, 0, Shift(arg, 1, 0))
// Shift down to account for the removed abstraction.
return Shift(substituted, -1, 0)
}
// Reduce performs normal order reduction on a De Bruijn expression.
func (r *NormalOrderReducer) Reduce() {
r.Emit(reducer.StartEvent)
@@ -59,7 +49,11 @@ func (r *NormalOrderReducer) Reduce() {
if fn, arg, ok := isViable(it.Current()); !ok {
it.Next()
} else {
it.Swap(betaReduce(fn, arg))
// Substitute arg for variable 0 in the body.
substituted := Substitute(fn.body, 0, Shift(arg, 1, 0))
// Shift down to account for the removed abstraction.
it.Swap(Shift(substituted, -1, 0))
r.Emit(reducer.StepEvent)
if _, _, ok := isViable(it.Parent()); ok {