Haskell Development Guide

Core Philosophy

1. Types are the design — Make illegal states unrepresentable. If the type checker accepts it, we would like it to be correct.
2. Purity by default — Side effects are explicit in the type system. IO is a feature, not a burden.
3. Composition over inheritance — Small, composable functions. Typeclasses for ad-hoc polymorphism.
4. Laziness as a tool — Enables elegant abstractions but demands awareness of space leaks.
5. Correctness first, then performance — Get it right, then profile, then optimize.
6. Keep it Simple — Purity and strong types (i.e. roughly Haskell2010) gives you the majority of Haskell's value. Avoid more advanced language features unless absolutely necessary.

Project Setup

Cabal (preferred with Nix)

mkdir my-project && cd my-project
cabal init --interactive

Minimal my-project.cabal:

cabal-version: 3.0
name:          my-project
version:       0.1.0.0
build-type:    Simple

common warnings
    ghc-options: -Wall -Werror -Wcompat -Widentities
                 -Wincomplete-record-updates
                 -Wincomplete-uni-patterns
                 -Wpartial-fields
                 -Wredundant-constraints

library
    import:           warnings
    exposed-modules:  MyProject
    build-depends:    base >= 4.17 && < 5
                    , text
                    , containers
                    , aeson
    hs-source-dirs:   src
    default-language:  Haskell2010
    default-extensions:
        DeriveGeneric
        DerivingStrategies
        LambdaCase
        ScopedTypeVariables

executable my-project
    import:           warnings
    main-is:          Main.hs
    build-depends:    base, my-project
    hs-source-dirs:   app
    default-language: Haskell2010

test-suite tests
    import:           warnings
    type:             exitcode-stdio-1.0
    main-is:          Main.hs
    build-depends:    base, my-project, hspec, QuickCheck
    hs-source-dirs:   test
    default-language: Haskell2010

Project Structure

my-project/
├── exe/
│   └── Main.hs              # Executable entry point (thin — delegates to library)
├── src/
│   ├── MyProject.hs          # Public API (re-exports)
│   ├── MyProject/
│   │   ├── Types.hs          # Core domain types
│   │   ├── App.hs            # Application monad, config
│   │   ├── DB.hs             # Database layer
│   │   ├── API.hs            # HTTP/API layer
│   │   └── Internal/         # Not exported — implementation details
│   │       └── Utils.hs
├── test/
│   ├── Main.hs
│   └── MyProject/
│       ├── TypesSpec.hs
│       └── DBSpec.hs
├── my-project.cabal
├── cabal.project             # Multi-package config, source-repository-packages

Essential GHC Extensions