# Haskell Dependencies

This note covers `07-haskell-deps/`, which builds a small Haskell program that depends on external libraries from the Haskell package set.

---

## 1. What This Example Adds

The earlier Haskell examples stayed close to `base` and local modules.

This example adds three common Haskell libraries:

- `aeson` for JSON decoding,
- `text` for string data in the parsed value, and
- `bytestring` for the raw input passed into the decoder.

That makes the example useful for two reasons:

- it shows how Cabal `build-depends` entries become Nix build inputs automatically, and
- it demonstrates a more realistic program shape than a pure string concatenation example.

---

## 2. The Important Point About Dependencies

The flake still uses the same Nix expression pattern as `05-haskell/`:

```nix
project = haskellPackages.callCabal2nix "mini-json" ./. { };
```

The external libraries are not listed again in `flake.nix`. They are declared in `mini-json.cabal`, and `callCabal2nix` translates that Cabal package
description into a Nix derivation.

That is the main teaching point: Cabal remains the source of truth for Haskell package dependencies, while the flake decides how the package is
exposed as a build, app, dev shell, and check.

---

## 3. The Library Function

The library exposes one function:

```haskell
greetFromJson :: ByteString -> Either String String
```

It decodes a JSON object with a `name` field and returns a greeting string. The executable wraps that function in a small CLI, and the test suite
checks both a valid input and an invalid one.

That keeps the example focused on dependency usage, not CLI design.

---

## 4. Commands to Try

```bash
cd 07-haskell-deps

nix develop
cabal run
cabal run -- '{"name":"flakes"}'
cabal test

nix build
./result/bin/mini-json '{"name":"flakes"}'

nix run . -- '{"name":"flakes"}'
nix flake check
```