Merge pull request langchain-ai#3795 from langchain-ai/nc/26dec/msg-p…

…laceholder-optional Implement optional message placeholder in js
PennYao · Dec 26, 2023 · 5dfef00 · 5dfef00
2 parents 1eca176 + c556eda
commit 5dfef00
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Showing 2 changed files with 42 additions and 5 deletions.
diff --git a/langchain-core/src/prompts/chat.ts b/langchain-core/src/prompts/chat.ts
@@ -75,22 +75,28 @@ export abstract class BaseMessagePromptTemplate<
  */
 export interface MessagesPlaceholderFields<T extends string> {
   variableName: T;
+  optional?: boolean;
 }
 
 /**
  * Class that represents a placeholder for messages in a chat prompt. It
  * extends the BaseMessagePromptTemplate.
  */
 export class MessagesPlaceholder<
-  // eslint-disable-next-line @typescript-eslint/no-explicit-any
-  RunInput extends InputValues = any
-> extends BaseMessagePromptTemplate<RunInput> {
+    // eslint-disable-next-line @typescript-eslint/no-explicit-any
+    RunInput extends InputValues = any
+  >
+  extends BaseMessagePromptTemplate<RunInput>
+  implements MessagesPlaceholderFields<Extract<keyof RunInput, string>>
+{
   static lc_name() {
     return "MessagesPlaceholder";
   }
 
   variableName: Extract<keyof RunInput, string>;
 
+  optional: boolean;
+
   constructor(variableName: Extract<keyof RunInput, string>);
 
   constructor(
@@ -108,16 +114,27 @@ export class MessagesPlaceholder<
     }
     super(fields);
     this.variableName = fields.variableName;
+    this.optional = fields.optional ?? false;
   }
 
   get inputVariables() {
     return [this.variableName];
   }
 
   validateInputOrThrow(
-    input: Array<unknown>,
+    input: Array<unknown> | undefined,
     variableName: Extract<keyof RunInput, string>
   ): input is BaseMessage[] {
+    if (this.optional && !input) {
+      return false;
+    } else if (!input) {
+      const error = new Error(
+        `Error: Field "${variableName}" in prompt uses a MessagesPlaceholder, which expects an array of BaseMessages as an input value. Received: undefined`
+      );
+      error.name = "InputFormatError";
+      throw error;
+    }
+
     let isInputBaseMessage = false;
 
     if (Array.isArray(input)) {
@@ -147,7 +164,7 @@ export class MessagesPlaceholder<
   ): Promise<BaseMessage[]> {
     this.validateInputOrThrow(values[this.variableName], this.variableName);
 
-    return values[this.variableName];
+    return values[this.variableName] ?? [];
   }
 }
 

diff --git a/langchain-core/src/prompts/tests/chat.test.ts b/langchain-core/src/prompts/tests/chat.test.ts
@@ -275,6 +275,26 @@ test("Test SimpleMessagePromptTemplate", async () => {
   expect(messages).toEqual([new HumanMessage("Hello Foo, I'm Bar")]);
 });
 
+test("Test MessagesPlaceholder optional", async () => {
+  const prompt = new MessagesPlaceholder({
+    variableName: "foo",
+    optional: true,
+  });
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  const messages = await prompt.formatMessages({} as any);
+  expect(messages).toEqual([]);
+});
+
+test("Test MessagesPlaceholder not optional", async () => {
+  const prompt = new MessagesPlaceholder({
+    variableName: "foo",
+  });
+  // eslint-disable-next-line @typescript-eslint/no-explicit-any
+  await expect(prompt.formatMessages({} as any)).rejects.toThrow(
+    'Error: Field "foo" in prompt uses a MessagesPlaceholder, which expects an array of BaseMessages as an input value. Received: undefined'
+  );
+});
+
 test("Test using partial", async () => {
   const userPrompt = new PromptTemplate({
     template: "{foo}{bar}",