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rainoftimerainoftime/pl-skills/monad-transformer/SKILL.md

monad-transformer

Skill17 starsChanged 9 months agoApache-2.0View on GitHub

Implements monad transformers for composing monads in functional languages.

The file

We split this skill into the rules it gives an agent

3 rules. About 0 are common, 3 are distinctive, and 0 are very specific to this project.

  • ≈0

    Common · estimate

    Many files like this have a rule of this kind.

  • ≈3

    Distinctive · estimate

    This author's own choices. Worth a look.

  • 0

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  • Common
  • Distinctive
  • Very specific
  1. Distinctive:

    Start with simple stacks (Reader + IO)

    line 40
  2. Distinctive:

    Use mtL for lifting operations

    line 41
  3. Distinctive:

    Consider monad-unlift for better ergonomics

    line 42

≈ “Common” is an estimate: at least 1 in 5 of the 350,495 skill files we read have a rule of the same kind, not the same words. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.

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Files
SKILL.md

Monad Transformer

Implements monad transformers - reusable layers that add capabilities to monads, enabling composition of multiple effects in functional programs.

When to Use This Skill

  • Combining state, error handling, IO, and other effects
  • Building layered monadic interfaces
  • Creating custom monad stacks for applications
  • Implementing embedded DSLs with multiple effects

What This Skill Does

  1. Transformer Library: Provides standard transformers (Reader, Writer, State, Error, IO)
  2. Stack Construction: Builds monad stacks from transformer layers
  3. Lift/Unlift: Implements monad lifting between transformer layers
  4. Typeclass Instances: Derives Functor, Applicative, Monad for transformer stacks

Key Concepts

ConceptDescription
Monad TransformerType constructor that adds capabilities to a monad
Monad StackComposition of multiple transformers
LiftingRunning actions from inner monads in outer context
MonadIOInterface for IO actions in transformer stacks

Tips

  • Start with simple stacks (Reader + IO)
  • Use mtL for lifting operations
  • Consider monad-unlift for better ergonomics
  • Order matters: inner monad first

Related Skills

  • effect-system - Effect tracking
  • algebraic-effects - Alternative effect composition
  • type-class-implementer - Type class implementation

Canonical References

ReferenceWhy It Matters
Grabmüller, "Monad Transformers: Step by Step" (2010)Comprehensive tutorial on building monad transformers
Elliott, Marlow, et al., "The Haskell Transformers Library"Standard library design for composable monad transformers
Kiselyov, "Eff: Functional Effects and Monads in OCaml" (2013+)Alternative approach to algebraic effects

Tradeoffs and Limitations

ApproachProsCons
TransformersComposable, reusableType complexity, lift fatigue
Algebraic EffectsSimpler effect syntaxLess mature tooling
Free MonadsPure, testablePerformance overhead

Assessment Criteria

CriterionWhat to Look For
CorrectnessAll typeclass laws satisfied
LiftingProper lift implementation
ComposabilityTransformers stack correctly
PerformanceMinimal overhead

Quality Indicators

✅ Good: Typeclass laws satisfied, proper lifting, stacks compose ⚠️ Warning: Partial typeclass support, some lift issues ❌ Bad: Type errors, incorrect behavior

Research Tools & Artifacts

Monad transformer implementations:

ToolLanguageWhat to Learn
mtlHaskellStandard transformers
transformersHaskellAlternative
CatsScalaScala integration
PurescriptPurescriptEff alternative

Research Frontiers

1. Effect Systems

  • Goal: Replace transformers with effects
  • Approach: Row polymorphism, effect handlers

Implementation Pitfalls

PitfallReal ConsequenceSolution
Lift cascadeToo many liftsUse monad-freer

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