Support --foo=bar syntax (#98)

Fixes #97

Relatively straightforward change. We do `head.split("=", 2) match` to
see if it contains any `=`, and if so treat the portion of the string
after the first `=` as the value. Added some unit test to cover both
success cases and failure cases: use with short names or with flags is
unsupported.

Author: lihaoyi

mainargs/src/TokenGrouping.scala

@@ -18,32 +18,53 @@ object TokenGrouping {
     }
 
     val flatArgs = flatArgs0.toList
-    val keywordArgMap = argSigs
+    def makeKeywordArgMap(getNames: ArgSig => Iterable[String]) = argSigs
       .collect {
         case (a, r: TokensReader.Simple[_]) if !a.positional => a
         case (a, r: TokensReader.Flag) => a
       }
-      .flatMap { x => (x.name.map("--" + _) ++ x.shortName.map("-" + _)).map(_ -> x) }
+      .flatMap { x => getNames(x).map(_ -> x) }
       .toMap[String, ArgSig]
 
+    lazy val keywordArgMap = makeKeywordArgMap(
+      x => x.name.map("--" + _) ++ x.shortName.map("-" + _)
+    )
+
+    lazy val longKeywordArgMap = makeKeywordArgMap(x => x.name.map("--" + _))
+
     @tailrec def rec(
         remaining: List[String],
         current: Map[ArgSig, Vector[String]]
     ): Result[TokenGrouping[B]] = {
       remaining match {
         case head :: rest =>
+
+          def lookupArgMap(k: String, m: Map[String, ArgSig]): Option[(ArgSig, mainargs.TokensReader[_])] = {
+            m.get(k).map(a => (a, a.reader))
+          }
+
           if (head.startsWith("-") && head.exists(_ != '-')) {
-            keywordArgMap.get(head) match {
-              case Some(cliArg: ArgSig) if cliArg.reader.isFlag =>
-                rec(rest, Util.appendMap(current, cliArg, ""))
-              case Some(cliArg: ArgSig) if !cliArg.reader.isLeftover =>
-                rest match {
-                  case next :: rest2 => rec(rest2, Util.appendMap(current, cliArg, next))
-                  case Nil =>
-                    Result.Failure.MismatchedArguments(Nil, Nil, Nil, incomplete = Some(cliArg))
+            head.split("=", 2) match{
+              case Array(first, second) =>
+                lookupArgMap(first, longKeywordArgMap) match {
+                  case Some((cliArg, _: TokensReader.Simple[_])) =>
+                    rec(rest, Util.appendMap(current, cliArg, second))
+
+                  case _ => complete(remaining, current)
                 }
 
-              case _ => complete(remaining, current)
+              case _ =>
+                lookupArgMap(head, keywordArgMap) match {
+                  case Some((cliArg, _: TokensReader.Flag)) =>
+                    rec(rest, Util.appendMap(current, cliArg, ""))
+                  case Some((cliArg, _: TokensReader.Simple[_])) =>
+                    rest match {
+                      case next :: rest2 => rec(rest2, Util.appendMap(current, cliArg, next))
+                      case Nil =>
+                        Result.Failure.MismatchedArguments(Nil, Nil, Nil, incomplete = Some(cliArg))
+                    }
+                  case _ => complete(remaining, current)
+                }
             }
           } else {
             positionalArgSigs.find(!current.contains(_)) match {

mainargs/test/src/EqualsSyntaxTests.scala

@@ -0,0 +1,55 @@
+package mainargs
+import utest._
+
+object EqualsSyntaxTests extends TestSuite {
+
+  object Main {
+    @main
+    def run(
+        @arg(short = 'f', doc = "String to print repeatedly")
+        foo: String,
+        @arg(name = "my-num", doc = "How many times to print string")
+        myNum: Int = 2,
+        @arg(doc = "Example flag")
+        bool: Flag
+    ) = {
+      foo * myNum + " " + bool.value
+    }
+  }
+
+  val tests = Tests {
+    test("simple") {
+      ParserForMethods(Main).runOrThrow(Array("--foo=bar", "--my-num=3")) ==>
+        "barbarbar false"
+    }
+    test("multipleEquals") {
+      // --foo=bar syntax still works when there's an `=` on the right
+      ParserForMethods(Main).runOrThrow(Array("--foo=bar=qux")) ==>
+        "bar=quxbar=qux false"
+    }
+    test("shortName") {
+      // -f=bar syntax doesn't work for short names
+      ParserForMethods(Main).runEither(Array("-f=bar")) ==>
+        Left(
+          """Missing argument: -f --foo <str>
+            |Unknown argument: "-f=bar"
+            |Expected Signature: run
+            |  -f --foo <str>  String to print repeatedly
+            |  --my-num <int>  How many times to print string
+            |  --bool          Example flag
+            |
+            |""".stripMargin)
+    }
+    test("notFlags") {
+      // -f=bar syntax doesn't work for flags
+      ParserForMethods(Main).runEither(Array("--foo=bar", "--bool=true")) ==>
+         Left("""Unknown argument: "--bool=true"
+                |Expected Signature: run
+                |  -f --foo <str>  String to print repeatedly
+                |  --my-num <int>  How many times to print string
+                |  --bool          Example flag
+                |
+                |""".stripMargin)
+    }
+  }
+}