CommandTree 0.3.2

CommandTree

Define CLI commands as F# discriminated unions. Get type-safe parsing, help generation, and fish completions from your types.

// From examples/ExampleCli/Program.fs

// my-cli task add "Buy groceries"
// my-cli task add "Buy groceries" high
// my-cli task                             ← runs list (CmdDefault)
// my-cli task complete 5
type TaskCommand =
    | [<Cmd("Add a new task")>] Add of title: string * priority: Priority option
    | [<Cmd("List all tasks"); CmdDefault>] List
    | [<Cmd("Complete a task")>] Complete of id: int
    | [<Cmd("Remove a task")>] Remove of id: int

// my-cli --verbose task add "Buy groceries"   ← global flag before command
// my-cli task add "Buy groceries" --verbose   ← global flag after command
type GlobalFlag =
    | [<Cmd("Enable verbose output")>] Verbose
    | [<Cmd("Set log level"); CmdEnv("LVL")>] LogLevel of string

type Command =
    | [<Cmd("Task management")>] Task of TaskCommand
    | [<Cmd("Run the test suite")>] Test
    | [<Cmd("Show full help")>] Help

let spec =
    CommandReflection.fromUnionWithGlobalsAndEnv<Command, GlobalFlag> "My CLI" "MYAPP"

match spec.Parse argv with
| Ok(globals, Task(Add(title, _))) -> printfn "Adding %s" title
| Ok(_, Help) -> printfn "%s" (CommandTree.helpWithGlobals spec.GlobalFlags spec.Tree "my-cli")
| Error(HelpRequested path) -> printfn "%s" (CommandTree.helpForPath spec.Tree path "my-cli")
| Error(UnknownCommand(input, _)) -> UI.fail $"Unknown command: %s{input}"
| _ -> ()

Installation

dotnet add package CommandTree

How It Works

Case names become kebab-case commands. Nested unions become subcommand groups. Fields become positional arguments.

// From examples/ExampleCli/Program.fs

// my-cli db migrate
// my-cli db                               ← runs status (CmdDefault)
type DbCommand =
    | [<Cmd("Run database migrations")>] Migrate
    | [<Cmd("Reset the database")>] Reset
    | [<Cmd("Show connection status"); CmdDefault>] Status

// my-cli deploy push staging
// my-cli deploy status prod
// my-cli deploy                           ← runs status with no env (CmdDefault)
type DeployCommand =
    | [<Cmd("Deploy to environment"); CmdCompletion("dev", "staging", "prod")>] Push of env: string
    | [<Cmd("Show deploy status"); CmdCompletion("dev", "staging", "prod"); CmdDefault>] Status of env: string option

// my-cli files tag v1.0 src/App.fs src/Lib.fs   ← list field collects 1+ remaining args
// my-cli files diff old.dll new.dll             ← multiple CmdFileCompletion with FieldIndex
type FilesCommand =
    | [<Cmd("Tag files with a label"); CmdFileCompletion>] Tag of label: string * files: string list
    | [<Cmd("Compare two DLLs"); CmdFileCompletion(FieldIndex = 0); CmdFileCompletion(FieldIndex = 1)>] Diff of
        oldDll: string *
        newDll: string

// my-cli job start build-assets 1024 true
// my-cli job status 550e8400-e29b-41d4-a716-446655440000
// my-cli job                              ← runs list (CmdDefault)
type JobCommand =
    | [<Cmd("Start a new job")>] Start of name: string * size: int64 * verbose: bool
    | [<Cmd("Check job status")>] Status of id: Guid
    | [<Cmd("List recent jobs"); CmdDefault>] List

// my-cli check --conf custom.json --strict --no-cache
type CheckFlag =
    | [<CmdFlag(Name = "conf", Short = "k")>] Config of string
    | [<Cmd("Enable strict checking")>] Strict
    | [<CmdEnvRaw("NO_CACHE")>] NoCache

type Command =
    | [<Cmd("Task management")>] Task of TaskCommand
    | [<Cmd("Database operations")>] Db of DbCommand
    | [<Cmd("Deployment")>] Deploy of DeployCommand
    | [<Cmd("File operations")>] Files of FilesCommand
    | [<Cmd("Job management")>] Job of JobCommand
    | [<Cmd("Run checks")>] Check of CheckFlag list
    | [<Cmd("Run the test suite")>] Test
    | [<Cmd("Show full help")>] Help

Mapping rules

Attributes

• [<Cmd("desc")>] sets help text; optional Name = "custom" overrides the command name
• [<CmdDefault>] marks the default subcommand when a group is invoked without arguments
• [<CmdCompletion("a", "b")>] provides fish shell completion values
• [<CmdFileCompletion>] enables file path completion in fish (multiple allowed per case with FieldIndex)
• [<CmdFlag(Name = "conf", Short = "k")>] overrides flag name or short flag on DU flag cases (optional — names are auto-derived)
• [<CmdEnv("SUFFIX")>] overrides the env var suffix for a flag (prefix still applied)
• [<CmdEnvRaw("VAR_NAME")>] sets the exact env var name, ignoring the prefix

Basic Usage

Full example: examples/ExampleCli/Program.fs

Without global options

open CommandTree

let tree = CommandReflection.fromUnion<Command> "My CLI"

[<EntryPoint>]
let main argv =
    match CommandTree.parse tree argv with
    | Ok cmd ->
        run cmd
        0
    | Error(HelpRequested path) ->
        printfn "%s" (CommandTree.helpForPath tree path "my-cli")
        0
    | Error e ->
        // UnknownCommand, InvalidArguments, AmbiguousArgument, UnknownFlag, DuplicateFlag
        UI.fail $"%A{e}"
        1

With global options and env vars

// From examples/ExampleCli/Program.fs
// GlobalFlag defined above in the intro
open CommandTree

let spec =
    CommandReflection.fromUnionWithGlobalsAndEnv<Command, GlobalFlag> "Example project management CLI" "EXAMPLE"

let cmdName = "example-cli"

[<EntryPoint>]
let main argv =
    match spec.Parse argv with
    | Ok(globals, cmd) ->
        if globals |> List.contains GlobalFlag.Verbose then
            UI.dimInfo "Verbose mode enabled"

        globals
        |> List.iter (function
            | GlobalFlag.LogLevel level -> UI.dimInfo $"Log level: %s{level}"
            | _ -> ())

        run spec.Tree cmdName cmd
        0
    | Error(HelpRequested path) ->
        if path.IsEmpty then
            printfn "%s" (CommandTree.helpWithGlobals spec.GlobalFlags spec.Tree cmdName)
        else
            printfn "%s" (CommandTree.helpForPath spec.Tree path cmdName)

        0
    | Error(UnknownCommand(input, path)) ->
        UI.fail $"Unknown command: %s{input}"
        printfn "%s" (CommandTree.helpForPath spec.Tree path cmdName)
        1
    | Error(InvalidArguments(cmd, msg)) ->
        UI.fail $"Invalid arguments for %s{cmd}: %s{msg}"
        1
    | Error(AmbiguousArgument(input, candidates)) ->
        let joined = String.concat ", " candidates
        UI.fail $"Ambiguous: '{input}' matches: {joined}"
        1
    | Error(UnknownFlag(flag, cmd, validFlags)) ->
        let joined = String.concat ", " validFlags
        UI.fail $"Unknown flag '%s{flag}' for command '%s{cmd}'. Valid flags: %s{joined}"
        1
    | Error(DuplicateFlag(flag, cmd)) ->
        UI.fail $"Flag '%s{flag}' provided more than once for command '%s{cmd}'"
        1

Global flags can appear anywhere in the arg list — before, after, or interleaved with command args. Duplicate flag names between global and command-level flags are rejected at tree construction time.

Reference

Parsing & Help

CommandTree.parse tree args              // Result<'Cmd, ParseError>
CommandTree.help tree path prefix        // Help text for one level
CommandTree.helpFull tree prefix         // Full recursive help
CommandTree.helpForPath tree path prefix // Help for a subcommand path
CommandTree.helpWithGlobals flags tree prefix // Help with global options section
CommandTree.format tree cmd path prefix  // Format command back to CLI string
CommandTree.findByPath tree path         // Navigate to a subtree
CommandTree.closestGroupPath tree args   // Deepest matching group path

Reflection

// Without global options
CommandReflection.fromUnion<'Cmd> "desc"                          // CommandTree<'Cmd>
CommandReflection.fromUnionWithEnv<'Cmd> "desc" "PREFIX"          // CommandTree<'Cmd> (with env vars)

// With global options (returns GlobalSpec with .Tree, .Parse, .GlobalFlags)
CommandReflection.fromUnionWithGlobals<'Cmd, 'G> "desc"           // GlobalSpec<'G, 'Cmd>
CommandReflection.fromUnionWithGlobalsAndEnv<'Cmd, 'G> "desc" "P" // GlobalSpec<'G, 'Cmd> (with env vars)

// Utilities
CommandReflection.formatCmd cmd              // Format command to CLI string
CommandReflection.caseName value             // Kebab-case name of union value
CommandReflection.toKebabCase "PascalCase"   // "pascal-case"
CommandReflection.parseFieldValue type str   // Result<obj option, string>
CommandReflection.formatFieldValue value     // Typed value to string

DU-Based Flags

Define flags as discriminated unions. No-field cases become boolean flags, single-field cases become value flags:

// From examples/ExampleCli/Program.fs

// example-cli check --conf custom.json --strict --no-cache
type CheckFlag =
    | [<CmdFlag(Name = "conf", Short = "k")>] Config of string
    | [<Cmd("Enable strict checking")>] Strict
    | [<CmdEnvRaw("NO_CACHE")>] NoCache

type Command =
    | [<Cmd("Run checks")>] Check of CheckFlag list

Short flags are auto-derived from the first letter (collision detection prevents duplicates). Use [<CmdFlag>] to override.

Env Var Binding

When an env prefix is configured, every DU flag case gets an auto-derived env var: PREFIX_SCREAMING_SNAKE_CASE.

// From examples/ExampleCli/Program.fs

type GlobalFlag =
    | [<Cmd("Enable verbose output")>] Verbose                     // env: EXAMPLE_VERBOSE
    | [<Cmd("Set log level"); CmdEnv("LVL")>] LogLevel of string   // env: EXAMPLE_LVL (suffix override)

type CheckFlag =
    | [<CmdEnvRaw("NO_CACHE")>] NoCache                            // env: NO_CACHE (exact name)

Resolution order: CLI flag > env var > absent. For boolean flags, env var values "true"/"1" mean present, "false"/"0"/unset mean absent.

Fish Completions

FishCompletions.generateContent tree "my-tool"  // Generate .fish content
FishCompletions.writeToFile tree "my-tool"      // Write to ~/.config/fish/completions/
FishCompletions.installHook "my-tool"           // Auto-update hook in conf.d

Supported Field Types

The library also includes Process (process execution helpers) and UI (colored terminal output) modules. See the API docs for details.

License

MIT