LLM4S Scala Functional LLM Interfaces
Functional LLM programming in Scala with categorical effect systems.
Installation
scala
// build.sbt libraryDependencies ++= Seq( "dev.zio" %% "zio" % "2.0.19", "dev.zio" %% "zio-json" % "0.6.2", "org.typelevel" %% "cats-effect" % "3.5.2", "co.fs2" %% "fs2-core" % "3.9.3" )
Core Abstractions
LLM as Effect
scala
import zio._
import zio.json._
// LLM call as effectful operation
trait LLMClient {
def complete(request: CompletionRequest): Task[CompletionResponse]
def stream(request: CompletionRequest): ZStream[Any, Throwable, String]
}
case class CompletionRequest(
model: String,
messages: List[Message],
temperature: Double = 0.7,
maxTokens: Option[Int] = None
)
case class Message(role: String, content: String)
case class CompletionResponse(
id: String,
content: String,
usage: Usage
)
case class Usage(promptTokens: Int, completionTokens: Int)
// JSON codecs
implicit val messageCodec: JsonCodec[Message] = DeriveJsonCodec.gen
implicit val requestCodec: JsonCodec[CompletionRequest] = DeriveJsonCodec.gen
implicit val responseCodec: JsonCodec[CompletionResponse] = DeriveJsonCodec.gen
OpenAI Client Implementation
scala
import zio.http._
class OpenAIClient(apiKey: String) extends LLMClient {
private val baseUrl = "https://api.openai.com/v1"
override def complete(request: CompletionRequest): Task[CompletionResponse] = {
for {
response <- Client.request(
Request.post(
URL.decode(s"$baseUrl/chat/completions").toOption.get,
Body.fromString(request.toJson)
).addHeader(Header.Authorization.Bearer(apiKey))
.addHeader(Header.ContentType(MediaType.application.json))
).provide(Client.default)
body <- response.body.asString
result <- ZIO.fromEither(body.fromJson[CompletionResponse])
.mapError(e => new Exception(s"Parse error: $e"))
} yield result
}
override def stream(request: CompletionRequest): ZStream[Any, Throwable, String] = {
// Streaming implementation
ZStream.fromZIO(complete(request)).map(_.content)
}
}
object OpenAIClient {
val layer: ZLayer[Any, Nothing, LLMClient] = ZLayer.succeed {
new OpenAIClient(sys.env("OPENAI_API_KEY"))
}
}
Categorical Patterns
Prompt as Functor
scala
import cats._
import cats.implicits._
// Prompt template with functor instance
case class Prompt[A](template: String, extract: String => A) {
def render(vars: Map[String, String]): String =
vars.foldLeft(template) { case (t, (k, v)) =>
t.replace(s"{$k}", v)
}
}
implicit val promptFunctor: Functor[Prompt] = new Functor[Prompt] {
def map[A, B](fa: Prompt[A])(f: A => B): Prompt[B] =
Prompt(fa.template, s => f(fa.extract(s)))
}
// Usage
val sentimentPrompt: Prompt[String] = Prompt(
"Analyze sentiment of: {text}\nSentiment:",
identity
)
val scoredPrompt: Prompt[Int] = sentimentPrompt.map {
case "positive" => 1
case "negative" => -1
case _ => 0
}
LLM Monad Transformer
scala
import cats.effect._
import cats.data._
// LLM effect as monad transformer
type LLMIO[A] = ReaderT[IO, LLMClient, A]
object LLMIO {
def complete(request: CompletionRequest): LLMIO[CompletionResponse] =
ReaderT(client => IO.fromFuture(IO(client.complete(request))))
def ask[A](prompt: String)(extract: String => A): LLMIO[A] =
complete(CompletionRequest(
model = "gpt-4o",
messages = List(Message("user", prompt))
)).map(r => extract(r.content))
def pure[A](a: A): LLMIO[A] = ReaderT.pure(a)
}
// Monadic composition
val pipeline: LLMIO[(String, List[String])] = for {
summary <- LLMIO.ask("Summarize this document...")(identity)
keywords <- LLMIO.ask(s"Extract keywords from: $summary")(
_.split(",").toList.map(_.trim)
)
} yield (summary, keywords)
Kleisli Composition
scala
import cats.data.Kleisli
import cats.effect.IO
type LLMKleisli[A, B] = Kleisli[IO, A, B]
// Composable LLM operations
def classify(text: String): IO[String] = IO.pure("positive")
def elaborate(category: String): IO[String] = IO.pure(s"Details about $category")
def format(details: String): IO[String] = IO.pure(s"Formatted: $details")
val classifyK: LLMKleisli[String, String] = Kleisli(classify)
val elaborateK: LLMKleisli[String, String] = Kleisli(elaborate)
val formatK: LLMKleisli[String, String] = Kleisli(format)
// Compose: classify >>> elaborate >>> format
val pipeline: LLMKleisli[String, String] =
classifyK >>> elaborateK >>> formatK
// Run
val result: IO[String] = pipeline.run("input text")
ZIO Integration
ZIO Service Pattern
scala
import zio._
// LLM as ZIO service
trait LLMService {
def generate(prompt: String): Task[String]
def generateStructured[A: JsonDecoder](prompt: String): Task[A]
def streamGenerate(prompt: String): ZStream[Any, Throwable, String]
}
object LLMService {
def generate(prompt: String): ZIO[LLMService, Throwable, String] =
ZIO.serviceWithZIO[LLMService](_.generate(prompt))
def generateStructured[A: JsonDecoder](prompt: String): ZIO[LLMService, Throwable, A] =
ZIO.serviceWithZIO[LLMService](_.generateStructured[A](prompt))
}
// Implementation
case class LLMServiceLive(client: LLMClient) extends LLMService {
override def generate(prompt: String): Task[String] =
client.complete(CompletionRequest(
model = "gpt-4o",
messages = List(Message("user", prompt))
)).map(_.content)
override def generateStructured[A: JsonDecoder](prompt: String): Task[A] =
generate(prompt).flatMap { content =>
ZIO.fromEither(content.fromJson[A])
.mapError(e => new Exception(s"Parse error: $e"))
}
override def streamGenerate(prompt: String): ZStream[Any, Throwable, String] =
client.stream(CompletionRequest(
model = "gpt-4o",
messages = List(Message("user", prompt))
))
}
object LLMServiceLive {
val layer: ZLayer[LLMClient, Nothing, LLMService] =
ZLayer.fromFunction(LLMServiceLive(_))
}
Error Handling
scala
// Typed errors
sealed trait LLMError
case class RateLimitError(retryAfter: Int) extends LLMError
case class InvalidRequestError(message: String) extends LLMError
case class ModelError(message: String) extends LLMError
// Error-aware operations
def safeGenerate(prompt: String): ZIO[LLMService, LLMError, String] =
LLMService.generate(prompt)
.mapError {
case e if e.getMessage.contains("rate_limit") =>
RateLimitError(60)
case e if e.getMessage.contains("invalid") =>
InvalidRequestError(e.getMessage)
case e =>
ModelError(e.getMessage)
}
// Retry with typed errors
def withRetry(prompt: String): ZIO[LLMService, LLMError, String] =
safeGenerate(prompt).retry(
Schedule.recurs(3) && Schedule.exponential(1.second)
)
Streaming with FS2
scala
import fs2._
import cats.effect._
// Streaming LLM responses
def streamCompletion(prompt: String): Stream[IO, String] = {
Stream.eval(IO.pure(prompt))
.through(tokenize)
.through(generateTokens)
}
def tokenize: Pipe[IO, String, String] =
_.flatMap(s => Stream.emits(s.split(" ")))
def generateTokens: Pipe[IO, String, String] =
_.evalMap(token => IO.sleep(50.millis) *> IO.pure(s"$token "))
// Consume stream
val program: IO[Unit] =
streamCompletion("Hello world")
.evalMap(token => IO(print(token)))
.compile
.drain
Tool Integration
scala
// Tool definition
case class Tool[A, B](
name: String,
description: String,
execute: A => Task[B]
)
// Tool-augmented LLM
trait ToolAugmentedLLM {
def generateWithTools[A: JsonDecoder](
prompt: String,
tools: List[Tool[_, _]]
): Task[A]
}
// Example tools
val searchTool = Tool[String, List[String]](
name = "search",
description = "Search the web",
execute = query => ZIO.succeed(List(s"Result for: $query"))
)
val calculatorTool = Tool[String, Double](
name = "calculate",
description = "Evaluate math expression",
execute = expr => ZIO.attempt(/* safe eval */ 0.0)
)
Categorical Guarantees
LLM4S provides:
- •Effect Safety: All LLM calls wrapped in IO/Task
- •Composability: Kleisli/ReaderT for pipeline composition
- •Type Safety: JSON codecs ensure structured output types
- •Error Handling: Typed errors with ZIO/Cats Effect
- •Streaming: FS2/ZStream for token-by-token processing