docs: Add subcommand example

1 files changed, 106 insertions, 1 deletions

diff --git a/README.md b/README.md
index 3281d91..d36715c 100644
--- a/README.md
+++ b/README.md
@@ -103,7 +103,7 @@ The result will contain an `args` field and a `positionals` field. `args` will h
 for each none positional parameter of your program. The name of the field will be the longest
 name of the parameter. `positionals` be a tuple with one field for each positional parameter.
 
-The fields in `args` and `psotionals` are typed. The type is based on the name of the value the
+The fields in `args` and `postionals` are typed. The type is based on the name of the value the
 parameter takes. Since `--number` takes a `usize` the field `res.args.number` has the type `usize`.
 
 Note that this is only the case because `clap.parsers.default` has a field called `usize` which
@@ -166,6 +166,111 @@ const std = @import("std");
 
 ```
 
+### Subcommands
+
+There is an option for `clap.parse` and `clap.parseEx` called `terminating_positional`. It allows
+for users of `clap` to implement subcommands in their cli application:
+
+```zig
+// These are our subcommands
+const SubCommands = enum {
+    help,
+    math,
+};
+
+const main_parsers = .{
+    .command = clap.parsers.enumeration(SubCommands),
+};
+
+// The parameters for main. Parameters for the subcommands are specified further down
+const main_params = clap.parseParamsComptime(
+    \\-h, --help  Display this help and exit.
+    \\<command>
+    \\
+);
+
+// To pass around arguments returned by clap, `clap.Result` and `clap.ResultEx` can be used to
+// get the return type of `clap.parse` and `clap.parseEx`
+const MainArgs = clap.ResultEx(clap.Help, &main_params, main_parsers);
+
+pub fn main() !void {
+    var gpa_state = std.heap.GeneralPurposeAllocator(.{}){};
+    const gpa = gpa_state.allocator();
+    defer _ = gpa_state.deinit();
+
+    var iter = try std.process.ArgIterator.initWithAllocator(gpa);
+    defer iter.deinit();
+
+    _ = iter.next();
+
+    var diag = clap.Diagnostic{};
+    var res = clap.parseEx(clap.Help, &main_params, main_parsers, &iter, .{
+        .diagnostic = &diag,
+        .allocator = gpa,
+
+        // Terminate the parsing of arguments after parsing the first positional (0 is passed here
+        // because parsed positionals are, like slices and arrays, indexed starting at 0).
+        //
+        // This will terminate the parsing after parsing the subcommand enum and leave `iter` not
+        // fully consumed. It can then be reused to parse the arguments for subcommands
+        .terminating_positional = 0,
+    }) catch |err| {
+        diag.report(std.io.getStdErr().writer(), err) catch {};
+        return err;
+    };
+    defer res.deinit();
+
+    if (res.args.help != 0)
+        std.debug.print("--help\n", .{});
+
+    const command = res.positionals[0] orelse return error.MissingCommand;
+    switch (command) {
+        .help => std.debug.print("--help\n", .{}),
+        .math => try mathMain(gpa, &iter, res),
+    }
+}
+
+fn mathMain(gpa: std.mem.Allocator, iter: *std.process.ArgIterator, main_args: MainArgs) !void {
+    // The parent arguments are not used it, but there are cases where it might be useful, so the
+    // example shows how to pass the arguments around.
+    _ = main_args;
+
+    // The parameters for the subcommand
+    const params = comptime clap.parseParamsComptime(
+        \\-h, --help  Display this help and exit.
+        \\-a, --add   Add the two numbers
+        \\-s, --sub   Subtract the two numbers
+        \\<isize>
+        \\<isize>
+        \\
+    );
+
+    // Here we pass the partially parsed argument iterator
+    var diag = clap.Diagnostic{};
+    var res = clap.parseEx(clap.Help, &params, clap.parsers.default, iter, .{
+        .diagnostic = &diag,
+        .allocator = gpa,
+    }) catch |err| {
+        diag.report(std.io.getStdErr().writer(), err) catch {};
+        return err;
+    };
+    defer res.deinit();
+
+    const a = res.positionals[0] orelse return error.MissingArg1;
+    const b = res.positionals[1] orelse return error.MissingArg1;
+    if (res.args.help != 0)
+        std.debug.print("--help\n", .{});
+    if (res.args.add != 0)
+        std.debug.print("added: {}\n", .{a + b});
+    if (res.args.sub != 0)
+        std.debug.print("subtracted: {}\n", .{a - b});
+}
+
+const clap = @import("clap");
+const std = @import("std");
+
+```
+
 ### `streaming.Clap`
 
 The `streaming.Clap` is the base of all the other parsers. It's a streaming parser that uses an