Add exists command

src/vm/commands/exists.ts

@@ -0,0 +1,51 @@
+import {Command, ParseContext, processPath, StrLVal, StrTerm, wrapInt} from "./command.js";
+import {LexInput} from "../lexer.js";
+import {ExecutionError, StrVM} from "../vm.js";
+import {Awaitable} from "../../util/types.js";
+import * as fs from 'node:fs';
+import {log} from "../../log.js";
+
+export type ExistsData = {
+    check: 'fs'|'var',
+    lexical: StrTerm,
+}
+
+export class Exists extends Command<ExistsData> {
+    async attemptParse(context: ParseContext): Promise<ExistsData> {
+        return {
+            check: context.popKeywordInSet(['fs', 'var']).value,
+            lexical: await context.popTerm(),
+        }
+    }
+
+    getDisplayName(): string {
+        return 'exists'
+    }
+
+    isParseCandidate(token: LexInput): boolean {
+        return this.isKeyword(token, 'exists')
+    }
+
+    execute(vm: StrVM, data: ExistsData): Awaitable<StrVM> {
+        const logger = log.getStreamLogger('cmd-exists')
+
+        return vm.replaceContextMatchingTerm(ctx => ({
+            override: async () => {
+                if ( data.check === 'fs' ) {
+                    const path = processPath(ctx.resolveString(data.lexical))
+
+                    logger.debug({ checkPath: path })
+                    return wrapInt(fs.existsSync(path) ? 1 : 0)
+                } else if ( data.check === 'var' ) {
+                    const lval: StrLVal = { term: 'variable', name: '$' + ctx.resolveString(data.lexical) }
+                    const val = ctx.resolve(lval)
+
+                    logger.debug({ checkVar: lval, val })
+                    return wrapInt(val ? 1 : 0)
+                }
+
+                throw new ExecutionError(`Existence operator not implemented: ${data.check}`)
+            },
+        }))
+    }
+}