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61 lines
1.9 KiB
Markdown
61 lines
1.9 KiB
Markdown
# How to write and use external functions
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> NOTE: this is experimental and subject to change. Expect to get your hands
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> dirty and everything to break every other Catala version. Only OCaml is
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> supported at the moment.
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## Write an interface file
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I.e. declare your function in a catala file, defining the module as external:
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> Module Prorata_external external
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```catala-metadata
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declaration prorata content collection money
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depends on amount content money,
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weights content collection decimal
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```
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## Get the expected function prototype
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To get the expected prototype the easiest at the moment is to write a dummy
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catala implementation and compile to OCaml (removing the `external` directive):
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> Module Prorata_external
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```catala
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declaration prorata content collection money
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depends on amount content money,
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weights content collection decimal
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equals [amount]
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```
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```shell-session
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$ clerk build _build/.../Prorata_external.ml
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```
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(beware the `_build/`, and the capitalisation of the module name)
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## Write the OCaml implementation
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Copy the obtained ml file besides the `.catala_en` file (adjusting
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capitalisation to match). Edit to replace the dummy implementation by your code.
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Refer to `runtimes/ocaml/runtime.mli` for what is available (especially the
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`Oper` module to manipulate the types).
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Keep the `register_module` at the end as is, it's needed for the toplevel to use
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the value (you would get `Failure("Could not resolve reference to Xxx")` during
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evaluation).
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## Compile and test
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`Clerk` should pick up the `.ml` file automatically. Make sure you cleaned up
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the dummy implementation version from step 2 (and declare as `external`) in the
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`.catala_en` file and that the `.ml` file with the same name sits besides it.
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Use the module as any other module (e.g. `> Using Prorata_external as M` then
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`M.prorata`).
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There is an example in `tests/test_modules/external_use.catala_en`.
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