# Haskell Shell with shellFor

This note covers `06-haskell-shellfor/`, which builds a local Haskell package and constructs the dev shell with `shellFor` instead of `mkShell`.

---

## 1. What `shellFor` Changes

`mkShell` is a generic shell constructor. You list tools manually.

`shellFor` is specific to Haskell package sets. It starts from one or more Haskell packages and builds a development environment around their
dependencies.

That means the shell tracks the package definition more closely. When the package's Haskell dependencies change, the shell changes with it.

---

## 2. Why the Package Set Override Exists

The example adds the local package to the Haskell package set first:

```nix
haskellPackages = pkgs.haskellPackages.override {
  overrides = final: _: {
    mini-shellfor = final.callCabal2nix "mini-shellfor" ./. { };
  };
};
```

`shellFor` expects packages from the package set it is working with. Defining `mini-shellfor` inside that set makes it available both as a normal
package output and as a package that `shellFor` can reference.

---

## 3. The Dev Shell

The shell itself is small:

```nix
devShells.${system}.default = haskellPackages.shellFor {
  packages = hp: [ hp.mini-shellfor ];

  nativeBuildInputs = [
    pkgs.cabal-install
    pkgs.haskell-language-server
  ];
};
```

`packages = hp: [ hp.mini-shellfor ];` tells `shellFor` which Haskell package should drive the environment. `nativeBuildInputs` adds the interactive
tools you still want on top.

---

## 4. Commands to Try

```bash
cd 06-haskell-shellfor

nix develop
cabal run
cabal test

nix build
./result/bin/mini-shellfor

nix run
nix flake check
```