From 4791c0212273904aae620175a1ca07aee2ac4bcf Mon Sep 17 00:00:00 2001
From: Brian Picciano <mediocregopher@gmail.com>
Date: Sun, 23 Jun 2019 12:55:11 -0600
Subject: [PATCH] mcfg: implement ParamDefaultOrRequired

---
 mcfg/mcfg_test.go | 20 ++++++++++++++++++++
 mcfg/param.go     | 12 ++++++++++++
 2 files changed, 32 insertions(+)

diff --git a/mcfg/mcfg_test.go b/mcfg/mcfg_test.go
index bbc7470..940ff6a 100644
--- a/mcfg/mcfg_test.go
+++ b/mcfg/mcfg_test.go
@@ -47,3 +47,23 @@ func TestPopulate(t *T) {
 		assert.Equal(t, 3, *c)
 	}
 }
+
+func TestParamDefaultOrRequired(t *T) {
+	{
+		cmp := new(mcmp.Component)
+		Int(cmp, "a", ParamDefaultOrRequired(0))
+		params := CollectParams(cmp)
+		assert.Equal(t, "a", params[0].Name)
+		assert.Equal(t, true, params[0].Required)
+		assert.Equal(t, new(int), params[0].Into)
+	}
+	{
+		cmp := new(mcmp.Component)
+		Int(cmp, "a", ParamDefaultOrRequired(1))
+		i := 1
+		params := CollectParams(cmp)
+		assert.Equal(t, "a", params[0].Name)
+		assert.Equal(t, false, params[0].Required)
+		assert.Equal(t, &i, params[0].Into)
+	}
+}
diff --git a/mcfg/param.go b/mcfg/param.go
index 26a405b..b5dd6a3 100644
--- a/mcfg/param.go
+++ b/mcfg/param.go
@@ -83,6 +83,18 @@ func ParamDefault(value interface{}) ParamOption {
 	}
 }
 
+// ParamDefaultOrRequired returns a ParamOption whose behavior depends on the
+// given value. If the given value is the zero value for its type, then this returns
+// ParamRequired(), otherwise this returns ParamDefault(value).
+func ParamDefaultOrRequired(value interface{}) ParamOption {
+	v := reflect.ValueOf(value)
+	zero := reflect.Zero(v.Type())
+	if v.Interface() == zero.Interface() {
+		return ParamRequired()
+	}
+	return ParamDefault(value)
+}
+
 // ParamUsage returns a ParamOption which sets the usage string on the Param.
 // This is used in some Sources, like SourceCLI, when displaying information
 // about available parameters.