str/src/vm/commands/prefix.ts

import {Command, ParseContext, StrTerm} from "./command.js";
import {LexInput} from "../lexer.js";
import {StrVM} from "../vm.js";
import {Awaitable} from "../../util/types.js";

export class Prefix extends Command<{ with: StrTerm }> {
    attemptParse(context: ParseContext): { with: StrTerm } {
        return {
            with: context.popTerm(),
        }
    }

    getDisplayName(): string {
        return 'prefix'
    }

    isParseCandidate(token: LexInput): boolean {
        return this.isKeyword(token, 'prefix')
    }

    execute(vm: StrVM, data: { with: StrTerm }): Awaitable<StrVM> {
        return vm.inPlace(ctx =>
            ctx.replaceSubject(sub =>
                sub.modify(s => `${ctx.resolveString(data.with)}${s}`)))
    }
}