nix-playgraound/notes/040-haskell-transformer-stack.md

Haskell Transformer Stacks

This note covers 37-haskell-transformer-stack/, which composes ReaderT, ExceptT, and Writer in one rollout workflow.

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1. Why a Stack Helps Here

The rollout workflow needs three independent capabilities:

• configuration from an environment,
• explicit business failures, and
• ordered audit output.

This example puts them together directly:

type App = ReaderT Env (ExceptT RolloutError (Writer [String]))

That is the core teaching point. The effect requirements live in one concrete stack, while the workflow stays readable.

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2. What the Workflow Actually Does

The stack is not there for decoration. The rollout code:

• reads cluster and policy settings,
• rejects restricted or oversized production rollouts, and
• records each successful step in an audit log.

That gives each transformer a concrete job.

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3. Why This Complements the Earlier Examples

Earlier notes introduced these pieces separately:

• ReaderT and Except in 10-haskell-effects/, and
• Writer in 31-haskell-writer-audit/.

This example shows the next practical step: combining them when one workflow needs all three.

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4. Commands to Try

cd 37-haskell-transformer-stack

nix develop
cabal run
cabal run -- api:production:4
cabal test

nix build
./result/bin/mini-stack api:production:4

nix run . -- api:production:4
nix flake check