Meet the Evaluations framework

// Evaluations
  import Evaluations

  struct BookTaggingEvaluation: Evaluation {
  
  }

// Optimization Target
  @Test("Book Tag Evaluations", .evaluates(evaluation, info: evaluationInfo))
  func evaluateBookTagging() async throws {
      let result = EvaluationContext.current.result
  
      let rangeMetric = BookTagEvaluationTests.evaluation.tagCount
      #expect(result.aggregateValue(.mean(of: rangeMetric)) >= 0.8)
  }

// BookTags.swift
  @Generable
  struct BookTags: Codable {
      @Guide(description: "Descriptive tags capturing themes, genres, moods, and topics from the summary", .count(3...8))
      var tags: [String]
  } snippet.

// BookTaggingEvaluation
  var dataset = ArrayLoader(samples: [
      ModelSample(prompt: "okay I am OBSESSED and I need everyone to read this RIGHT NOW...",
                  expected: BookTags(tags: ["classic", "romance", "wit", "regency"])),

      ModelSample(prompt: "Read this in one sitting between midnight and 4am and I cannot...",
                  expected: BookTags(tags: ["classic", "gothic", "horror", "vampire", "suspense"])),
  ])
  
  // Or load your whole library:
  var dataset = ArrayLoader(samples:
      Book.sampleBooks.map { book in
          ModelSample(prompt: book.review, expected: BookTags(tags: book.tags))
      }
  )

// Synthesizing more inputs
  let samples: [ModelSample<String>] = [
      ModelSample(prompt: "The largest planet in our solar system...", expected: "Jupiter."),
      ModelSample(prompt: "The capital of Thailand...", expected: "Bangkok."),
      ModelSample(prompt: "Swift is...", expected: "a powerful programming language."),
      ModelSample(prompt: "All those moments will be lost in time...", expected: "Like tears in rain.")
  ]
  
  for try await sample in samples.makeSamples(
      """
      Generate diverse sentence completions about the listed topics:
        - The Solar System
        - World Capitals 
      """,
      targetCount: 1000) {
          samples.append(sample)
  }

let wordCount = Metric("WordCount")

  Evaluator { _, subject in
      for tag in subject.value.tags {
          if tag.contains(" ") {
              return wordCount.failing(rationale: "Tag \(tag) contains multiple words")
          }
      }
      return wordCount.passing()
  }

  let hasGenreTag = Metric("HasGenreTag")
  
  Evaluator { _, subject in
      let tags = subject.value.tags.map { $0.lowercased() }
      let knownGenres = await BookTaggingService.knownGenres
      for tag in tags {
          if knownGenres.contains(tag) {
              return hasGenreTag.passing(rationale: "Matched \(tag)")
          }
      }
      return hasGenreTag.failing() 
  }

let tagCount = Metric("TagCount")

  var evaluators: Evaluators {

      // Tag count is within the required 3–8 range
      Evaluator { _, subject in 
          let count = subject.value.tags.count
          if (count >= 3 && count <= 8) {
              return tagCount.passing(rationale: "\(count) tags")
          } 
          return tagCount.failing(rationale: "Got \(count) tags, expected 3–8")
      }
  }

let tagCount = Metric("TagCount")
  let tagTotal = Metric("TagTotal")
  
  func aggregateMetrics(using aggregator: inout MetricsAggregator) {
      aggregator.computeMean(of: tagCount)
      aggregator.group("Distribution of Tag Totals") { aggregator in
          aggregator.computeStandardDeviation(of: tagTotal)
          aggregator.computeMean(of: tagTotal)
          aggregator.computeVariance(of: tagTotal)
      }
  }

// BookTaggingService.swift
  let instructions = Instructions {
      """
      You are a librarian and literary analyst. Given a reader's
      freeform summary of a book they read — describing their
      thoughts, feelings, and what stood out — generate a set of
      descriptive tags reflected in the summary.

      Rules:
       - Return between 3 and 8 tags.
       - Tags should be lowercase, concise (single word or hyphenated), and descriptive.
       - Tags should include the book's genre, chosen from the included list of known genres.
  
      Known Genres:
       - \(Self.knownGenres.joined(separator: ", "))
      """
  }

ModelJudgeEvaluator(
      "TagQuality",
      scale: .numeric([
          4: "Tags are relevant and helpful for browsing",
          3: "Mostly relevant, one tag too vague or generic",
          2: "Several tags are wrong or generic",
          1: "Unhelpful or irrelevant"
      ]),   
      judge: PrivateCloudComputeLanguageModel()
  )

// BookTaggingEvaluation.swift
  ScoreDimension(
      "Relevance",
      description: """
          Whether each tag describes a quality, theme, or tone
          of the book itself rather than incidental details or
          the reader's personal reactions.
          """,
      scale: .numeric([
          4: "Every tag describes the book itself",
          3: "Most tags describe the book",
          2: "Some tags describe personal reactions",
          1: "Tags don't meaningfully describe the book"
      ])    
  )
  // Define `usefulness` the same way as a second ScoreDimension.

// BookTaggingEvaluation.swift
  var evaluators: Evaluators {

      Evaluator {  }  

      Evaluator {  }

      Evaluator {  }
  
      ModelJudgeEvaluator(
          judge: PrivateCloudComputeLanguageModel(),
          dimensions: [relevance, usefulness]
      )
  }

// BookTaggingEvaluation.swift
  ModelJudgeEvaluator(
      judge: PrivateCloudComputeLanguageModel(),
      dimensions: [relevance, usefulness],
      prompt: ModelJudgePrompt( 
          instructions: """
              You are evaluating tags generated for a personal book-tracking app where users
              organize their library by browsing and filtering tags.
              """,
          evaluationTarget: { value in
              "\(value.tags.count) Generated tags: " + value.tags.joined(separator: ", ")
          },
          reference: { input, _ in 
              let expectedTags = input.expected?.tags.joined(separator: ", ")
              return ["Expected Tags": expectedTags ?? "No expected tags defined"]
          }
      )
  )