From d903bd9a13e790cf42c84c3dc59bf89ffeae1d80 Mon Sep 17 00:00:00 2001
From: Elizabeth Hunt <elizabeth@simponic.xyz>
Date: Fri, 28 Feb 2025 21:21:10 -0700
Subject: add max depth term

---
 src/interpreter/interpreter.ts | 37 +++++++++++++++++++++++--------------
 1 file changed, 23 insertions(+), 14 deletions(-)

diff --git a/src/interpreter/interpreter.ts b/src/interpreter/interpreter.ts
index 4599c46..e90b8ce 100644
--- a/src/interpreter/interpreter.ts
+++ b/src/interpreter/interpreter.ts
@@ -9,6 +9,8 @@ import {
 
 export class InvalidLambdaTermError extends Error {}
 
+export class MaxRecursionDepthError extends Error {}
+
 export type DebrujinAbstraction = {
   abstraction: {
     param: string;
@@ -32,7 +34,7 @@ export type DebrujinifiedLambdaTerm =
 
 export const debrujinify = (
   term: LambdaTerm,
-  symbolTable: SymbolTable,
+  symbolTable: SymbolTable
 ): DebrujinifiedLambdaTerm => {
   if (isVariable(term)) {
     if (!symbolTable.has(term)) {
@@ -65,14 +67,14 @@ export const debrujinify = (
   }
 
   throw new InvalidLambdaTermError(
-    `Invalid lambda term: ${JSON.stringify(term)}`,
+    `Invalid lambda term: ${JSON.stringify(term)}`
   );
 };
 
 export const substitute = (
   inTerm: DebrujinifiedLambdaTerm,
   index: number,
-  withTerm: DebrujinifiedLambdaTerm,
+  withTerm: DebrujinifiedLambdaTerm
 ): DebrujinifiedLambdaTerm => {
   if ("index" in inTerm) {
     if (inTerm.index > index) {
@@ -108,13 +110,13 @@ export const substitute = (
   }
 
   throw new InvalidLambdaTermError(
-    `Invalid lambda term: ${JSON.stringify(inTerm)}`,
+    `Invalid lambda term: ${JSON.stringify(inTerm)}`
   );
 };
 
 export const adjustIndices = (
   term: DebrujinifiedLambdaTerm,
-  delta: number,
+  delta: number
 ): DebrujinifiedLambdaTerm => {
   if ("index" in term) {
     return {
@@ -146,13 +148,17 @@ export const adjustIndices = (
   }
 
   throw new InvalidLambdaTermError(
-    `Invalid lambda term: ${JSON.stringify(term)}`,
+    `Invalid lambda term: ${JSON.stringify(term)}`
   );
 };
 
 export const betaReduce = (
   term: DebrujinifiedLambdaTerm,
+  maxDepth: number,
 ): DebrujinifiedLambdaTerm => {
+  if (maxDepth === 0) {
+    throw new MaxRecursionDepthError("max recursion depth identified");
+  }
   if ("index" in term) {
     return term;
   }
@@ -162,7 +168,7 @@ export const betaReduce = (
 
     return {
       abstraction: {
-        body: betaReduce(body),
+        body: betaReduce(body, maxDepth - 1),
         param,
       },
     };
@@ -170,7 +176,9 @@ export const betaReduce = (
 
   if ("application" in term) {
     const { left } = term.application;
-    const args = term.application.args.map(betaReduce);
+    const args = term.application.args.map((term) =>
+      betaReduce(term, maxDepth - 1)
+    );
 
     return args.reduce((acc: DebrujinifiedLambdaTerm, x) => {
       if ("abstraction" in acc) {
@@ -194,7 +202,7 @@ export const betaReduce = (
   }
 
   throw new InvalidLambdaTermError(
-    `Invalid lambda term: ${JSON.stringify(term)}`,
+    `Invalid lambda term: ${JSON.stringify(term)}`
   );
 };
 
@@ -214,7 +222,7 @@ export const emitDebrujin = (term: DebrujinifiedLambdaTerm): string => {
   }
 
   throw new InvalidLambdaTermError(
-    `Invalid lambda term: ${JSON.stringify(term)}`,
+    `Invalid lambda term: ${JSON.stringify(term)}`
   );
 };
 
@@ -225,7 +233,7 @@ export const emitNamed = (term: DebrujinifiedLambdaTerm): string => {
 
   if ("abstraction" in term) {
     return `(λ (${term.abstraction.param}) . ${emitNamed(
-      term.abstraction.body,
+      term.abstraction.body
     )})`;
   }
 
@@ -241,18 +249,19 @@ export const emitNamed = (term: DebrujinifiedLambdaTerm): string => {
 export const interpret = (
   term: string,
   symbolTable = new SymbolTable(),
-  allowUnderscores = false,
+  maxDepth = 15,
+  allowUnderscores = false
 ): DebrujinifiedLambdaTerm => {
   const ast = parse(term, allowUnderscores);
   const debrujined = debrujinify(ast, symbolTable);
 
   let prev = debrujined;
-  let next = betaReduce(prev);
+  let next = betaReduce(prev, maxDepth);
 
   while (emitDebrujin(prev) !== emitDebrujin(next)) {
     // alpha equivalence
     prev = next;
-    next = betaReduce(prev);
+    next = betaReduce(prev, maxDepth);
   }
   return next;
 };
-- 
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