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# OCaml {#sec-language-ocaml}

     

       
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## User guide {#sec-language-ocaml-user-guide}

     

       
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OCaml libraries are available in attribute sets of the form `ocaml-ng.ocamlPackages_X_XX` where X is to be replaced with the desired compiler version. For example, ocamlgraph compiled with OCaml 4.12 can be found in `ocaml-ng.ocamlPackages_4_12.ocamlgraph`. The compiler itself is also located in this set, under the name `ocaml`.

     

       
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If you don't care about the exact compiler version, `ocamlPackages` is a top-level alias pointing to a recent version of OCaml.

     

       
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OCaml applications are usually available top-level, and not inside `ocamlPackages`. Notable exceptions are build tools that must be built with the same compiler version as the compiler you intend to use like `dune` or `ocaml-lsp`.

     

       
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To open a shell able to build a typical OCaml project, put the dependencies in `buildInputs` and add `ocamlPackages.ocaml` and `ocamlPackages.findlib` to `nativeBuildInputs` at least.

     

       
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For example:

     

       
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```nix

     

       
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let

     

       
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 pkgs = import <nixpkgs> {};

     

       
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 # choose the ocaml version you want to use

     

       
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 ocamlPackages = pkgs.ocaml-ng.ocamlPackages_4_12;

     

       
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in

     

       
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pkgs.mkShell {

     

       
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  # build tools

     

       
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  nativeBuildInputs = with ocamlPackages; [ ocaml findlib dune_2 ocaml-lsp ];

     

       
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  # dependencies

     

       
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  buildInputs = with ocamlPackages; [ ocamlgraph ];

     

       
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}

     

       
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```

     

       
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## Packaging guide {#sec-language-ocaml-packaging}

     

       
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OCaml libraries should be installed in `$(out)/lib/ocaml/${ocaml.version}/site-lib/`. Such directories are automatically added to the `$OCAMLPATH` environment variable when building another package that depends on them or when opening a `nix-shell`.

     

       
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Given that most of the OCaml ecosystem is now built with dune, nixpkgs includes a convenience build support function called `buildDunePackage` that will build an OCaml package using dune, OCaml and findlib and any additional dependencies provided as `buildInputs` or `propagatedBuildInputs`.