Add exists command

HELP.md

@@ -446,6 +446,12 @@ Assuming the current subject is a valid single data document, convert it `from`
 Supports: `php`|`json`
 Example: `{"a": 1}` -> `convert json php` -> `['a' => 1]`
 
+#### `exists <fs|var> <name>`
+Checks whether the given file or variable exists.
+Supports: `fs` for filesystem checks, `var` for variable names
+Example: `$a = 1` -> `exists var a` -> `1`
+Example: `exists fs fake-file.txt` -> `0`
+
 #### User-specific setup files
 The file `~/.str.rc` is automatically executed when `str` starts up.
 You can use this to define a user-specific environment.

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}`)
+            },
+        }))
+    }
+}

src/vm/commands/index.ts

@@ -66,6 +66,7 @@ import {Times} from "./times.js";
 import {Divide} from "./divide.js";
 import {Abs} from "./abs.js";
 import {Rand} from "./rand.js";
+import {Exists} from "./exists.js";
 
 export type Commands = Command<CommandData>[]
 export const commands: Commands = [
@@ -83,6 +84,7 @@ export const commands: Commands = [
     new Each,
     new Edit,
     new Enclose,
+    new Exists,
     new Exit,
     new Flatten,
     new From,