added some tests for usage generation

spec/usage_spec.lua

@@ -0,0 +1,87 @@
+local argparse = require "argparse"
+
+describe("tests related to usage message generation", function()
+   it("creates correct usage message for empty parser", function()
+      local parser = argparse.parser "foo"
+      assert.equal(parser:prepare():get_usage(), "Usage: foo")
+   end)
+
+   it("creates correct usage message for arguments", function()
+      local parser = argparse.parser "foo"
+      parser:argument "first"
+      parser:argument "second-and-third" {
+         args = 2
+      }
+      parser:argument "maybe-fourth" {
+         args = "?"
+      }
+      parser:argument "others" {
+         args = "*"
+      }
+      assert.equal(
+         [=[Usage: foo <first> <second-and-third> <second-and-third> [<maybe-fourth>] [<others>] ...]=],
+         parser:prepare():get_usage()
+      )
+   end)
+
+   it("creates correct usage message for options", function()
+      local parser = argparse.parser "foo"
+      parser:flag "-q" "--quiet"
+      parser:option "--from" {
+         count = 1,
+         target = "server"
+      }
+      parser:option "--config"
+      assert.equal(
+         [=[Usage: foo [-q] --from <server> [--config <config>]]=],
+         parser:prepare():get_usage()
+      )
+   end)
+
+   it("creates correct usage message for commands", function()
+      local parser = argparse.parser "foo"
+      parser:flag "-q" "--quiet"
+      local run = parser:command "run"
+      run:option "--where"
+      assert.equal(
+         [=[Usage: foo [-q] [<command>] ...]=],
+         parser:prepare():get_usage()
+      )
+   end)
+
+   describe("usage generation can be customized", function()
+      it("uses message provided by user", function()
+         local parser = argparse.parser "foo" {
+            usage = "Usage: obvious"
+         }
+         parser:flag "-q" "--quiet"
+         assert.equal(
+            [=[Usage: obvious]=],
+            parser:prepare():get_usage()
+         )
+      end)
+
+      it("uses per-option message provided by user", function()
+         local parser = argparse.parser "foo"
+         parser:flag "-q" "--quiet" {
+            usage = "[-q | --quiet]"
+         }
+         assert.equal(
+            [=[Usage: foo [-q | --quiet]]=],
+            parser:prepare():get_usage()
+         )
+      end)
+
+      it("uses argnames provided by user", function()
+         local parser = argparse.parser "foo"
+         parser:argument "inputs" {
+            args = "1-2",
+            argname = "<input>"
+         }
+         assert.equal(
+            [=[Usage: foo <input> [<input>]]=],
+            parser:prepare():get_usage()
+         )
+      end)
+   end)
+end)