diff --git a/src/lib/guesser.ts b/src/lib/guesser.ts
index 6ba23d9..c0afca6 100644
--- a/src/lib/guesser.ts
+++ b/src/lib/guesser.ts
@@ -1,9 +1,13 @@
 import { assertExists } from "https://deno.land/std@0.224.0/assert/assert_exists.ts";
 
-import { enumerate, range } from "./utils.ts";
+import { Awaitable, enumerate, range } from "./utils.ts";
 import { Dict } from "./dict.ts";
 import { GuessResult } from "./game/game.ts";
 
+export interface Guessing {
+	guess(try_: (guess: string, known: string) => Awaitable<GuessResult>): Promise<GuessResult | null>;
+}
+
 type Knowledge = {
 	letter: string;
 	at: Set<number>;
@@ -11,7 +15,7 @@ type Knowledge = {
 	not_at: Set<number>;
 };
 
-export class Gueser {
+export class Guesser implements Guessing {
 	length;
 	dict;
 	informations;
@@ -25,9 +29,9 @@ export class Gueser {
 		}
 	}
 
-	async next_guess(operation: (guess: string, known: string) => GuessResult | Promise<GuessResult>) {
+	async guess(try_: (guess: string, known: string) => Awaitable<GuessResult>) {
 		const res = this.expected_result();
-		if (res.completed) return operation(res.result, res.result);
+		if (res.completed) return await try_(res.result, res.result);
 
 		const words = [...this.dict.words.values()];
 		const scored = words.map((word) => [this.info_score_for(word), word] as const);
@@ -35,8 +39,8 @@ export class Gueser {
 		const bests = scored.filter(([s]) => s === best_score).map(([_, word]) => word);
 		const guess = pick_random_common_word(bests);
 
-		const result = await operation(guess, res.result);
-		if (result.kind === "success") return guess;
+		const result = await try_(guess, res.result);
+		if (result.kind === "success") return result;
 		for (const [index, info] of enumerate(result.informations)) {
 			const letter = guess[index];
 			if (info.kind === "there") this.learn_letter_at(letter, index);
@@ -46,6 +50,7 @@ export class Gueser {
 				this.learn_letter_not_at(letter, index);
 			}
 		}
+		return null;
 	}
 
 	learn_does_not_exist(letter: string) {