Add optionalValueLazy (#421)

Author: ajalt

CHANGELOG.md

@@ -1,7 +1,9 @@
 # Changelog
+
 ## Unreleased
 ### Added
 - Added `MordantHelpFormatter.renderAttachedOptionValue` that you can override to change how option values are shown, e.g. if you want option to show as `--option <value>` instead of `--option=<value>`. ([#416](https://github.com/ajalt/clikt/issues/416))
+- Added `option().optionalValueLazy{}`, which work like `optionalValue()` but the default value is computed lazily. ([#381](https://github.com/ajalt/clikt/issues/381))
 
 ## 4.0.0
 ### Added

clikt/src/commonMain/kotlin/com/github/ajalt/clikt/parameters/options/TransformEach.kt

@@ -114,6 +114,8 @@ fun <ValueT> NullableOption<ValueT, ValueT>.varargValues(
  * To avoid ambiguity when combined with [arguments][argument], you can set [acceptsUnattachedValue]
  * to `false` to require that the option's value be specified like `--foo=bar` rather than `--foo bar`.
  *
+ * You can use [optionalValueLazy] if you need to reference other parameters.
+ *
  * ## Example
  *
  * ```
@@ -137,3 +139,39 @@ fun <ValueT : Any> NullableOption<ValueT, ValueT>.optionalValue(
         it.firstOrNull() ?: default
     }.copy(acceptsUnattachedValue = acceptsUnattachedValue)
 }
+
+
+/**
+ * Allow this option to be specified with or without an explicit value.
+ *
+ * If the option is specified on the command line without a value, [default] will be called and its
+ * return value can be used.
+ *
+ * To avoid ambiguity when combined with [arguments][argument], you can set [acceptsUnattachedValue]
+ * to `false` to require that the option's value be specified like `--foo=bar` rather than `--foo bar`.
+ *
+ * You can use [optionalValue] if you don't need to reference other parameters.
+ *
+ * ## Example
+ *
+ * ```
+ * val log by option().optionalValueLazy{"verbose"}.default("none")
+ *
+ * > ./tool --log=debug
+ * log level == debug
+ *
+ * > ./tool --log
+ * log level == verbose
+ *
+ * > ./tool
+ * log level == none
+ * ```
+ */
+fun <ValueT : Any> NullableOption<ValueT, ValueT>.optionalValueLazy(
+    acceptsUnattachedValue: Boolean = true,
+    default: () -> ValueT,
+): NullableOption<ValueT, ValueT> {
+    return transformValues(0..1) {
+        it.firstOrNull() ?: default()
+    }.copy(acceptsUnattachedValue = acceptsUnattachedValue)
+}

clikt/src/commonTest/kotlin/com/github/ajalt/clikt/parameters/VarargOptionsTest.kt

@@ -4,10 +4,7 @@ import com.github.ajalt.clikt.core.MissingArgument
 import com.github.ajalt.clikt.core.NoSuchOption
 import com.github.ajalt.clikt.core.subcommands
 import com.github.ajalt.clikt.parameters.arguments.argument
-import com.github.ajalt.clikt.parameters.options.default
-import com.github.ajalt.clikt.parameters.options.option
-import com.github.ajalt.clikt.parameters.options.optionalValue
-import com.github.ajalt.clikt.parameters.options.varargValues
+import com.github.ajalt.clikt.parameters.options.*
 import com.github.ajalt.clikt.parameters.types.int
 import com.github.ajalt.clikt.testing.TestCommand
 import com.github.ajalt.clikt.testing.parse
@@ -52,6 +49,18 @@ class VarargOptionsTest {
         C().parse(argv)
     }
 
+    @Test
+    fun optionalValueLazy() {
+        class C : TestCommand() {
+            val o by option().int().default(1)
+            val p by option().int().optionalValueLazy { o }
+            override fun run_() {
+                p shouldBe 2
+            }
+        }
+        C().parse("--o=2 --p")
+    }
+
     @Test
     fun varargValues() = forAll(
         row("", null),

docs/options.md

@@ -217,7 +217,8 @@ variables](#values-from-environment-variables).
 
 ### Options with an Optional Value
 
-You can create options that take zero or one values with [`optionalValue`][optionalValue].
+You can create options that take zero or one values with [`optionalValue`][optionalValue] or
+[`optionalValueLazy`][optionalValueLazy].
 
 === "Example"
     ```kotlin
@@ -1244,6 +1245,7 @@ val opt: Pair<Int, Int> by option("-o", "--opt")
 [mutuallyExclusiveOptions]:    api/clikt/com.github.ajalt.clikt.parameters.groups/mutually-exclusive-options.html
 [option]:                      api/clikt/com.github.ajalt.clikt.parameters.options/option.html
 [optionalValue]:               api/clikt/com.github.ajalt.clikt.parameters.options/optional-value.html
+[optionalValueLazy]:           api/clikt/com.github.ajalt.clikt.parameters.options/optional-value-lazy.html
 [pair]:                        api/clikt/com.github.ajalt.clikt.parameters.options/pair.html
 [parameter-types]:             parameters.md#parameter-types
 [PrintMessage]:                api/clikt/com.github.ajalt.clikt.core/-print-message/index.html