feat: Introduce prompt type option for Agent, Basic LLM Chain, and QA Chain nodes (#8697)

Signed-off-by: Oleg Ivaniv <me@olegivaniv.com>
Co-authored-by: Michael Kret <michael.k@radency.com>

packages/@n8n/nodes-langchain/utils/helpers.ts

@@ -1,6 +1,7 @@
-import type { IExecuteFunctions } from 'n8n-workflow';
+import { NodeConnectionType, type IExecuteFunctions, NodeOperationError } from 'n8n-workflow';
 import { BaseChatModel } from 'langchain/chat_models/base';
 import { BaseChatModel as BaseChatModelCore } from '@langchain/core/language_models/chat_models';
+import type { BaseOutputParser } from '@langchain/core/output_parsers';
 
 export function getMetadataFiltersValues(
 	ctx: IExecuteFunctions,
@@ -18,6 +19,48 @@ export function getMetadataFiltersValues(
 }
 
 // TODO: Remove this function once langchain package is updated to 0.1.x
+// eslint-disable-next-line @typescript-eslint/no-duplicate-type-constituents
 export function isChatInstance(model: any): model is BaseChatModel | BaseChatModelCore {
 	return model instanceof BaseChatModel || model instanceof BaseChatModelCore;
 }
+
+export async function getOptionalOutputParsers(
+	ctx: IExecuteFunctions,
+): Promise<Array<BaseOutputParser<unknown>>> {
+	let outputParsers: BaseOutputParser[] = [];
+
+	if (ctx.getNodeParameter('hasOutputParser', 0, true) === true) {
+		outputParsers = (await ctx.getInputConnectionData(
+			NodeConnectionType.AiOutputParser,
+			0,
+		)) as BaseOutputParser[];
+	}
+
+	return outputParsers;
+}
+
+export function getPromptInputByType(options: {
+	ctx: IExecuteFunctions;
+	i: number;
+	promptTypeKey: string;
+	inputKey: string;
+}) {
+	const { ctx, i, promptTypeKey, inputKey } = options;
+	const prompt = ctx.getNodeParameter(promptTypeKey, i) as string;
+
+	let input;
+	if (prompt === 'auto') {
+		input = ctx.evaluateExpression('{{ $json["chatInput"] }}', i) as string;
+	} else {
+		input = ctx.getNodeParameter(inputKey, i) as string;
+	}
+
+	if (input === undefined) {
+		throw new NodeOperationError(ctx.getNode(), 'No prompt specified', {
+			description:
+				"Expected to find the prompt in an input field called 'chatInput' (this is what the chat trigger node outputs). To use something else, change the 'Prompt' parameter",
+		});
+	}
+
+	return input;
+}