nix-playgraound/notes/009-haskell-dependencies.md

Haskell Dependencies

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

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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.

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2. The Important Point About Dependencies

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

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.

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3. The Library Function

The library exposes one function:

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.

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4. Commands to Try

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