Add if command

HELP.md

@@ -381,6 +381,14 @@ str %> call $myFooReplacer bar
 ```
 
 
+### Control Structures
+
+#### `if <cond> <lambda>`
+Execute the given lambda if `cond` is truthy. Cond may be a lambda, in which case it is executed.
+A value is truthy UNLESS it is the literal empty string (`''`) or the integer `0`.
+Example: `foo` -> `if (contains foo) (suffix bar)` -> `foobar`
+
+
 ### Misc
 
 #### `lipsum <num> <word|line|para>`

src/vm/commands/if.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 IfData = {
+	cond: StrTerm,
+	callable: StrTerm,
+}
+
+export class If extends Command<IfData> {
+	async attemptParse(context: ParseContext): Promise<IfData> {
+		return {
+			cond: await context.popTerm(),
+			callable: await context.popTerm(),
+		}
+	}
+
+	isParseCandidate(token: LexInput): boolean {
+		return this.isKeyword(token, 'if')
+	}
+
+	getDisplayName(): string {
+		return 'if'
+	}
+
+	async execute(vm: StrVM, data: IfData): Promise<StrVM> {
+		return vm.replaceContextFromChild(async (childVM, ctx) => {
+			let cond = ctx.resolveRequired(data.cond)
+
+			// If `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 empty/falsy, so do not execute the body.
+				return
+			}
+
+			const callable = ctx.resolveLambda(data.callable)
+
+			await (new Call).execute(childVM, {
+				callable,
+				params: [],
+			})
+		})
+	}
+}

src/vm/commands/index.ts

@@ -54,6 +54,7 @@ import {Script} from "./script.js";
 import {Take} from "./take.js";
 import {Group} from "./group.js";
 import {Flatten} from "./flatten.js";
+import {If} from "./if.js";
 
 export type Commands = Command<CommandData>[]
 export const commands: Commands = [
@@ -74,6 +75,7 @@ export const commands: Commands = [
     new Group,
     new Help,
     new History,
+    new If,
     new Indent,
     new InFile,
     new Join,