From 3d5f1f70ff37a942a27ace0a1aa1627638418c14 Mon Sep 17 00:00:00 2001
From: Tangent Wantwight <tangent128@gmail.com>
Date: Fri, 20 Oct 2023 19:06:01 -0400
Subject: [PATCH] Propagate errors up to root of script evaluation

---
 TODO            |  2 +-
 src/lib/card.ts |  5 +++--
 src/vm.ts       | 50 ++++++++++++++++++++++++++++++-------------------
 src/words.ts    | 13 +++++++++++--
 4 files changed, 46 insertions(+), 24 deletions(-)

diff --git a/TODO b/TODO
index 7b0fbe4..9c4ace7 100644
--- a/TODO
+++ b/TODO
@@ -2,6 +2,7 @@
 
   - card fields (vars maybe not needed yet?)
     - set field command
+      - command buffer in VM
       - formatter helper for debug page
         - unit test
       - command
@@ -16,7 +17,6 @@
 
 - impl backslash escapes
 - eval words
-- error handling
 - arithmatic
 - subst
 - standard argv -> name+switches+positionals helper
diff --git a/src/lib/card.ts b/src/lib/card.ts
index 43d5e10..79336ed 100644
--- a/src/lib/card.ts
+++ b/src/lib/card.ts
@@ -1,5 +1,5 @@
 import { Vm } from "../vm";
-import { TextPiece } from "../words";
+import { ProcResult, TextPiece } from "../words";
 import { getOpt } from "./options";
 
 /**
@@ -18,7 +18,8 @@ export type CardVm = Vm<{
   card: Card;
 }>;
 
-export function GetField(vm: CardVm, argv: TextPiece[]): TextPiece {
+export function GetField(vm: CardVm, argv: TextPiece[]): ProcResult {
   const [{ error }, fieldName] = getOpt(argv, { $min: 1, $max: 1 });
+  if (error) return { error };
   return { text: vm.card.fields[fieldName] ?? error ?? "" };
 }
diff --git a/src/vm.ts b/src/vm.ts
index 9221955..e88e725 100644
--- a/src/vm.ts
+++ b/src/vm.ts
@@ -1,13 +1,11 @@
 import {
-  AsHtml,
   AsText,
-  BareWord,
   Concat,
-  HtmlWord,
+  ErrorResult,
   InterpolatedPiece,
+  ProcResult,
   Script,
   TextPiece,
-  TextWord,
   Word,
 } from "./words";
 
@@ -23,7 +21,7 @@ export type ScriptType = "action" | "render";
 export type Proc<Context> = (
   state: Vm<Context>,
   argv: TextPiece[]
-) => TextPiece;
+) => ProcResult;
 
 /**
  * State for running a script in.
@@ -40,7 +38,7 @@ export type Vm<Context = {}> = {
 function evaluateWord<Context>(
   state: Vm<Context>,
   word: Word | InterpolatedPiece
-): TextPiece {
+): TextPiece | ErrorResult {
   if ("bare" in word || "text" in word || "html" in word) {
     return word;
   } else if ("variable" in word) {
@@ -48,11 +46,16 @@ function evaluateWord<Context>(
   } else if ("script" in word) {
     return runNoctl(state, word.script);
   } else {
-    return (
-      word.pieces
-        .map((piece) => evaluateWord(state, piece))
-        .reduce(Concat, null) ?? { text: "" }
-    );
+    let fullWord = null;
+    for (const piece of word.pieces) {
+      const result = evaluateWord(state, piece);
+      if ("error" in result) {
+        return result;
+      } else {
+        fullWord = Concat(fullWord, result);
+      }
+    }
+    return fullWord ?? { text: "" };
   }
 }
 
@@ -65,21 +68,30 @@ function evaluateWord<Context>(
 export function runNoctl<Context>(
   state: Vm<Context>,
   script: Script,
-  onReturn?: (word: TextPiece) => void
-): TextPiece {
-  let returnWord: TextPiece = { text: "" };
+  onReturn?: (word: TextPiece | ErrorResult) => void
+): TextPiece | ErrorResult {
+  let returnWord: TextPiece | ErrorResult = { text: "" };
+
+  for (const command of script) {
+    const argv: TextPiece[] = [];
+    for (const word of command) {
+      const processedWord = evaluateWord(state, word);
+      if ("error" in processedWord) {
+        onReturn?.(processedWord);
+        return processedWord;
+      } else {
+        argv.push(processedWord);
+      }
+    }
 
-  script.forEach((command) => {
-    const argv = command.map((word) => evaluateWord(state, word));
     const name = AsText(argv[0]);
     if (name in state.commands) {
       returnWord = state.commands[name](state, argv);
     } else {
-      // TODO: implement error propagation
-      returnWord = { html: `<b>UNKNOWN COMMAND: ${AsHtml(argv[0])}</b>` };
+      returnWord = { error: `Unknown Command: ${name}` };
     }
     onReturn?.(returnWord);
-  });
+  }
 
   return returnWord;
 }
diff --git a/src/words.ts b/src/words.ts
index d4503d0..c46e79f 100644
--- a/src/words.ts
+++ b/src/words.ts
@@ -1,4 +1,4 @@
-import { escapeHtml } from './helpers';
+import { escapeHtml } from "./helpers";
 
 /**
  * A word whose value is text with provenance- this literal value appeared in the source code,
@@ -32,11 +32,18 @@ export type HtmlWord = {
   html: string;
 };
 
+export type ErrorResult = {
+  error: string;
+};
+
 export type TextPiece = BareWord | TextWord | HtmlWord;
+
 export type VariablePiece = { variable: string };
 export type ScriptPiece = { script: Script };
 export type InterpolatedPiece = TextPiece | VariablePiece | ScriptPiece;
 
+export type ProcResult = TextPiece | ErrorResult;
+
 /**
  * A word whose value needs to be determined by evaluating some combination of variable and command
  * substitutions, and concatenating the results with any literal spans.
@@ -56,13 +63,15 @@ export function AsText(word: TextPiece): string {
     return "";
   }
 }
-export function AsHtml(word: TextPiece): string {
+export function AsHtml(word: TextPiece | ErrorResult): string {
   if ("bare" in word) {
     return escapeHtml(word.bare);
   } else if ("text" in word) {
     return escapeHtml(word.text);
   } else if ("html" in word) {
     return word.html;
+  } else if ("error" in word) {
+    return '<span style="color: red;">' + escapeHtml(word.error) + "</span>";
   } else {
     return "";
   }