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anil.recoil.org
1opam-version: "2.0"
2maintainer: "Gabriel Radanne <drupyog@zoho.com>"
3authors: [ "Thomas Gazagnaire"
4 "Anil Madhavapeddy"
5 "Dave Scott"
6 "Thomas Leonard"
7 "Gabriel Radanne" ]
8homepage: "https://github.com/mirage/functoria"
9bug-reports: "https://github.com/mirage/functoria/issues"
10dev-repo: "git+https://github.com/mirage/functoria.git"
11doc: "https://mirage.github.io/functoria/"
12license: "ISC"
13tags: ["org:mirage"]
14
15build: [
16 ["jbuilder" "subst" "-p" name] {dev}
17 ["jbuilder" "build" "-p" name "-j" jobs]
18]
19depends: [
20 "ocaml" {>= "4.02" & < "4.12"}
21 "jbuilder" {>= "1.0+beta10"}
22 "cmdliner" {>= "0.9.8"}
23 "fmt"
24]
25conflicts: [
26 "functoria" {< "2.0.0"}
27]
28synopsis: "A DSL to organize functor applications"
29description: """
30[](https://travis-ci.org/mirage/functoria)
31[](https://mirage.github.io/functoria/index.html)
32
33## What is this for?
34
35Functoria is a DSL to describe a set of modules and functors, their types and how to apply them in order to produce a complete application.
36
37The main use case is mirage. See the [mirage][] repository for details.
38
39## How to write a configuration file?
40
41There are numerous examples of configuration files in [mirage-skeleton][]. Most of them should be fairly general and understandable, even outside the context of mirage. We can distinguish two parts in a `config.ml`: Defining new modules and using them.
42
43In order to define a new module, we use the `foreign` function. Among its various arguments, it takes a module name and a type. The type is assembled with the DSL's combinators and the `@->` operator, which symbols a functor arrow.
44
45```ocaml
46let main = foreign "Unikernel.Main" (console @-> job)
47```
48
49Here, we declare the functor `Unikernel.Main` that takes a module that should be a `console` and returns a module that is a `job`. It is up to the user to ensure that the declaration matches the implementation (or be punished by a compiler error later on). If the declaration is correct, everything that follows will be.
50
51We can now use this declaration:
52
53```ocaml
54let () = register "console" [main $ default_console]
55```
56
57Here, we register a new application with the `register` function. This function should only be called once and takes as argument the name of the application and a list of jobs. We use the `$` operator to apply the functor `main` (aka `Unikernel.Main`) to the default console.
58
59Now that everything is ready, you can use the `configure` subcommand!
60
61### What is a job?
62
63A job is a module containing a function `start`. This function will receive one argument per functor argument and one per dependency, in this order. `foreign` assumes the function `start` returns `unit`.
64
65### Defining new keys
66
67A key is composed of:
68
69- _name_ : The name of the value in the program.
70- _description_ : How it should be displayed/serialized.
71- _stage_ : Is the key available only at runtime, at configure time or both?
72- _documentation_ : It is not optional so you should really write it.
73
74Consider a multilingual application: we want to pass the default language as a parameter. We will use a simple string, so we can use the predefined description `Key.Desc.string`. We want to be able to define it both at configure and run time, so we use the stage `` `Both``. This gives us the following code:
75
76```ocaml
77let lang_key =
78 let doc = Key.Doc.create
79 ~doc:"The default language for the application." [ "l" ; "lang" ]
80 in
81 Key.create ~doc ~stage:`Both ~default:"en" "language" Key.Desc.string
82```
83
84Here, we defined both a long option `--lang` and a short one `-l` (the format is similar to the one used by [Cmdliner][cmdliner]).
85In the application code, the value is retrieved with `Key_gen.language ()`.
86
87The option is also documented in the `--help` option for both the `configure` subcommand (at configure time) and `./my_application` (at startup time).
88
89```
90 -l VAL, --lang=VAL (absent=en)
91 The default language for the application.
92```
93
94[cmdliner]: http://erratique.ch/software/cmdliner
95
96### Using switching keys
97
98We can do much more with keys: we can use them to switch implementation at configure time. Imagine we want to completely change some implementation based on the language. Finns are special snowflakes, they deserve their special application!
99
100First, we have to compute a boolean value from `lang`:
101
102```ocaml
103let is_fi = Key.(pure ((=) "fi") $ value lang_key)
104```
105
106We can use the `if_impl` combinator to choose between two implementations depending on the value of the key:
107
108```ocaml
109let dynamic_storage =
110 if_impl is_fi
111 finnish_implementation
112 not_finnish_implementation
113```
114
115This distinction will be visible using the `describe` subcommand and a dot diagram is available with the `--dot` option!
116
117## Internals
118
119### Phases
120
121Configuration is separated into phases:
122
1231. Specialized DSL keys
124 The specialized DSL's keys (along with functoria's keys) are resolved.
1252. Compilation and dynlink of the config file.
1263. Registering.
127 When the `register` function is called, the list of jobs is recorded and
128 immediately transformed into a graph.
1294. Switching keys and tree evaluation.
130 The switching keys are the keys inside the [If].
131 Those keys are resolved and the graph is simplified. At this point,
132 the actual modules used are fully known.
133 Note: for the `describe` command, Only _partial_ evaluation is done, which
134 means decision nodes are resolved only if the value was given on the command
135 line, disregarding default values.
1365. Full Key resolution.
137 Once the actual modules are known, we can resolve all the keys and figure out
138 libraries and packages.
1396. Dependency handling, configuration and code emission.
140
141Phases 1. to 4. are also applied for the `clean` command.
142
143
144
145[mirage]: https://github.com/mirage/mirage
146[mirage-skeleton]: https://github.com/mirage/mirage-skeleton"""
147url {
148 src:
149 "https://github.com/mirage/functoria/releases/download/2.1.0/functoria-2.1.0.tbz"
150 checksum: [
151 "sha256=83fa056e52fe1fdc996d68ad2ce50f490d44e4b9cdd67e8c88a05753d99e70ec"
152 "md5=4541047f19a2842df17b9ffb5badbb3e"
153 ]
154}
155available: opam-version >= "2.2.0"
156flags: deprecated