Add unless command

HELP.md

@@ -388,6 +388,11 @@ Execute the given lambda if `cond` is truthy. Cond may be a lambda, in which cas
 A value is truthy UNLESS it is the literal empty string (`''`) or the integer `0`.
 Example: `foo` -> `if (contains foo) (suffix bar)` -> `foobar`
 
+#### `unless <cond> <lambda>`
+Inversion of `if`.
+Example: `foo` -> `unless (contains foo) (suffix bar)` -> `foo`
+
+
 
 ### Misc
 

src/vm/commands/index.ts

@@ -55,6 +55,7 @@ import {Take} from "./take.js";
 import {Group} from "./group.js";
 import {Flatten} from "./flatten.js";
 import {If} from "./if.js";
+import {Unless} from "./unless.js";
 
 export type Commands = Command<CommandData>[]
 export const commands: Commands = [
@@ -109,6 +110,7 @@ export const commands: Commands = [
     new Trim,
     new Undo,
     new Unique,
+    new Unless,
     new Unquote,
     new Upper,
     new Word,

src/vm/commands/unless.ts

@@ -0,0 +1,56 @@
+import {LexInput} from "../lexer.js";
+import {Command, ParseContext, StrTerm} from "./command.js";
+import {StrVM} from "../vm.js";
+import {Call} from "./call.js";
+
+export type UnlessData = {
+	cond: StrTerm,
+	callable: StrTerm,
+}
+
+export class Unless extends Command<UnlessData> {
+	async attemptParse(context: ParseContext): Promise<UnlessData> {
+		return {
+			cond: await context.popTerm(),
+			callable: await context.popTerm(),
+		}
+	}
+
+	isParseCandidate(token: LexInput): boolean {
+		return this.isKeyword(token, 'unless')
+	}
+
+	getDisplayName(): string {
+		return 'unless'
+	}
+
+	async execute(vm: StrVM, data: UnlessData): Promise<StrVM> {
+		return vm.replaceContextFromChild(async (childVM, ctx) => {
+			let cond = ctx.resolveRequired(data.cond)
+
+			// Unless `cond` is a lambda, then call it before evaluating its truthiness.
+			if ( cond.term === 'lambda' ) {
+				await childVM.runInChild(async (lambdaVM, lambdaCtx) => {
+					await (new Call).execute(lambdaVM, {
+						callable: cond,
+						params: [],
+					})
+
+					cond = lambdaCtx.getSubject()
+				})
+			}
+
+			if ( (cond.term !== 'string' && cond.term !== 'int') || cond.value ) {
+				// The string was truthy, so do not execute the body.
+				return
+			}
+
+			const callable = ctx.resolveLambda(data.callable)
+
+			await (new Call).execute(childVM, {
+				callable,
+				params: [],
+			})
+		})
+	}
+}