From 0f365758e11ca1669e31324bc6f530186d7c950a Mon Sep 17 00:00:00 2001
From: Eugen Wissner <belka@caraus.de>
Date: Tue, 30 May 2017 20:20:20 +0200
Subject: [PATCH] Add optional fieldnames for Pair

---
 source/tanya/typecons.d | 78 +++++++++++++++++++++++++++++++++++++----
 1 file changed, 72 insertions(+), 6 deletions(-)

diff --git a/source/tanya/typecons.d b/source/tanya/typecons.d
index cf09d52..62e9df4 100644
--- a/source/tanya/typecons.d
+++ b/source/tanya/typecons.d
@@ -5,6 +5,9 @@
 /**
  * Type constructors.
  *
+ * This module contains templates that allow to build new types from the
+ * available ones.
+ *
  * Copyright: Eugene Wissner 2017.
  * License: $(LINK2 https://www.mozilla.org/en-US/MPL/2.0/,
  *                  Mozilla Public License, v. 2.0).
@@ -15,17 +18,70 @@ module tanya.typecons;
 import std.meta;
 
 /**
- * $(D_PSYMBOL Pair) can store two heterogeneous objects. The objects can be
- * accessed than by the index. The objects can by accessed by index as
- * $(D_INLINECODE obj[0]) and $(D_INLINECODE obj[1]).
+ * $(D_PSYMBOL Pair) can store two heterogeneous objects.
+ *
+ * The objects can by accessed by index as $(D_INLINECODE obj[0]) and
+ * $(D_INLINECODE obj[1]) or by optional names (e.g.
+ * $(D_INLINECODE obj.first)).
+ *
+ * $(D_PARAM Specs) contains a list of object types and names. First
+ * comes the object type, then an optional string containing the name.
+ * If you want the object be accessible only by its index (`0` or `1`),
+ * just skip the name.
+ *
+ * Params:
+ *  Specs = Field types and names.
  */
-template Pair(Field1, Field2)
+template Pair(Specs...)
 {
-    /// Field types.
-    alias Types = AliasSeq!(Field1, Field2);
+    template parseSpecs(int fieldCount, Specs...)
+    {
+        static if (Specs.length == 0)
+        {
+            alias parseSpecs = AliasSeq!();
+        }
+        else static if (is(Specs[0]) && fieldCount < 2)
+        {
+            static if (is(typeof(Specs[1]) == string))
+            {
+                alias parseSpecs
+                    = AliasSeq!(Specs[0],
+                                parseSpecs!(fieldCount + 1, Specs[2 .. $]));
+            }
+            else
+            {
+                alias parseSpecs
+                    = AliasSeq!(Specs[0],
+                                parseSpecs!(fieldCount + 1, Specs[1 .. $]));
+            }
+        }
+        else
+        {
+            static assert(false, "Invalid argument: " ~ Specs[0].stringof);
+        }
+    }
 
     struct Pair
     {
+        /// Field types.
+        alias Types = parseSpecs!(0, Specs);
+
+        static assert(Types.length == 2, "Invalid argument count.");
+
+        // Create field aliases.
+        static if (is(typeof(Specs[1]) == string))
+        {
+            mixin("alias " ~ Specs[1] ~ " = expand[0];");
+        }
+        static if (is(typeof(Specs[2]) == string))
+        {
+            mixin("alias " ~ Specs[2] ~ " = expand[1];");
+        }
+        else static if (is(typeof(Specs[3]) == string))
+        {
+            mixin("alias " ~ Specs[3] ~ " = expand[1];");
+        }
+
         /// Represents the values of the $(D_PSYMBOL Pair) as a list of values.
         Types expand;
 
@@ -38,4 +94,14 @@ unittest
 {
     static assert(is(Pair!(int, int)));
     static assert(!is(Pair!(int, 5)));
+
+    static assert(is(Pair!(int, "first", int)));
+    static assert(is(Pair!(int, "first", int, "second")));
+    static assert(is(Pair!(int, "first", int)));
+
+    static assert(is(Pair!(int, int, "second")));
+    static assert(!is(Pair!("first", int, "second", int)));
+    static assert(!is(Pair!(int, int, int)));
+
+    static assert(!is(Pair!(int, "first")));
 }