71 lines
1.8 KiB
Markdown
71 lines
1.8 KiB
Markdown
# Haskell Dependencies
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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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---
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## 1. What This Example Adds
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The earlier Haskell examples stayed close to `base` and local modules.
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This example adds three common Haskell libraries:
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- `aeson` for JSON decoding,
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- `text` for string data in the parsed value, and
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- `bytestring` for the raw input passed into the decoder.
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That makes the example useful for two reasons:
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- it shows how Cabal `build-depends` entries become Nix build inputs automatically, and
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- it demonstrates a more realistic program shape than a pure string concatenation example.
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---
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## 2. The Important Point About Dependencies
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The flake still uses the same Nix expression pattern as `05-haskell/`:
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```nix
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project = haskellPackages.callCabal2nix "mini-json" ./. { };
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```
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The external libraries are not listed again in `flake.nix`. They are declared in `mini-json.cabal`, and `callCabal2nix` translates that Cabal package
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description into a Nix derivation.
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That is the main teaching point: Cabal remains the source of truth for Haskell package dependencies, while the flake decides how the package is
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exposed as a build, app, dev shell, and check.
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---
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## 3. The Library Function
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The library exposes one function:
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```haskell
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greetFromJson :: ByteString -> Either String String
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```
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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
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checks both a valid input and an invalid one.
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That keeps the example focused on dependency usage, not CLI design.
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---
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## 4. Commands to Try
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```bash
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cd 07-haskell-deps
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nix develop
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cabal run
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cabal run -- '{"name":"flakes"}'
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cabal test
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nix build
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./result/bin/mini-json '{"name":"flakes"}'
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nix run . -- '{"name":"flakes"}'
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nix flake check
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```
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