# Haskell Effects with ReaderT and Except

This note covers `10-haskell-effects/`, which models application logic with an environment, explicit errors, and a small effect stack.

---

## 1. What the Stack Represents

The example uses:

- `ReaderT Env` for read-only configuration, and
- `Except AppError` for failures that belong to the domain.

That is an important intermediate step because it separates three things cleanly:

- configuration,
- business logic, and
- error handling.

---

## 2. Why the Functions Use Constraints

The library functions are written against `MonadReader Env` and `MonadError AppError` constraints rather than a concrete stack type.

That keeps the functions reusable. They say what capabilities they need, not exactly which monad stack must provide them.

The concrete stack still exists:

```haskell
type App = ReaderT Env (Except AppError)
```

But the function signatures stay more flexible and easier to test.

---

## 3. Commands to Try

```bash
cd 10-haskell-effects

nix develop
cabal run
cabal run -- haskell
cabal test

nix build
./result/bin/mini-effects haskell

nix run . -- haskell
nix flake check
```