Model Context Protocol (MCP) tool integration and JSON-RPC dispatch for polytropos open records and variants.

Architectural Overview & Rationale

The MCP Schema Problem

In standard Model Context Protocol server implementations, defining tools requires tedious manual duplication: 1. Declaring a nominal parameter data type. 2. Deriving or writing an OpenAPI / JSON Schema definition. 3. Writing a parser that inspects the tool name string and runs the appropriate decoder. 4. Writing an error-prone dispatcher branch that invokes the handler.

The Polytropos Solution

polytropos-mcp collapses this entire pipeline into row types: * Tool Schemas: A tool's parameters are just an open record row type r. recordToTool reifies r into an OpenAPI schema directly via OpenApi. * Tool Call Variant: The entire suite of registered tools forms an open sum variant Variant s, where constructor tags match tool names and constructor payloads match parameter rows. * \(O(1)\) Dispatch: Tool calls are parsed into Variant s and dispatched using eliminateUniform, compiling to an indirect function call jump table without string comparisons or branch cascades. * Dynamic Extensibility: New tools can be added to the server by simply widening the variant with widenVariant.

When to Use This Module

• When exposing polytropos data pipelines, compilers, or database query engines to AI coding assistants (such as Claude, Gemini, or custom agent frameworks) via the standard Model Context Protocol.

Worked Example

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE OverloadedRecordDot #-}
{-# LANGUAGE TypeApplications #-}

import Data.Proxy (Proxy (..))
import Data.Text (Text)
import Polytropos
import Polytropos.MCP

-- 1. Define parameter rows for tools:
type SearchArgs = '["query" ':= Text, "limit" ':= Int]
type EchoArgs   = '["message" ':= Text]

-- 2. Define the tool registry as an open variant:
type MyTools = Variant
  '[ "search" ':= SearchArgs
   , "echo"   := EchoArgs
   ]

-- 3. Generate tool manifests with OpenAPI schemas:
toolList :: [McpTool]
toolList =
  [ recordToTool @"search" "Search indexed documents" (Proxy @SearchArgs)
  , recordToTool @"echo"   "Echo back a message"      (Proxy @EchoArgs)
  ]

-- 4. Dispatch tool calls using O(1) jump tables:
handleCall :: MyTools -> IO [McpContent]
handleCall = eliminateUniform REC
  { search = \args -> pure [TextContent ("Found hits for: " <> args.query)]
  , echo   = \args -> pure [TextContent ("Echo: " <> args.message)]
  }