rdmd --eval and --loop prints last @statement

changelog/rdmd-writeln.dd

@@ -0,0 +1,46 @@
+In rdmd `--eval` and `--loop` the last statement without effect will be printed if prefixed with `@`
+
+`--eval` and `--loop` are two convenience features of `rdmd` that allow quick experimentation with D code.
+
+$(CONSOLE
+rdmd --eval='"hello"'
+$(RED Error): Error: found } when expecting ; following statement
+rdmd --eval='"hello";'
+$(RED Error): "hello" has no effect
+rdmd --eval='@"hello"'
+hello
+)
+
+A few more examples of `rdmd` in action:
+
+$(H4 Use `rdmd` as your calculator)
+
+$(CONSOLE
+rdmd --eval="@2 + 2"
+4
+)
+
+$(CONSOLE
+rdmd --eval="@2.pow(4)"
+16
+)
+
+$(CONSOLE
+rdmd --eval="auto a = 10; auto b = [a, a + 1]; @b"
+[10, 11]
+)
+
+$(H4 Select the first column in a CSV file)
+
+$(CONSOLE
+rdmd --loop='@line.splitter(",").take(1)'
+16
+)
+
+$(H4 Use `rdmd` as drop-in for Unix tools)
+
+$(CONSOLE
+seq 5 | rdmd --loop='@stdin.byLine.drop(2).joiner(newline)'
+4
+5
+)

rdmd.d

@@ -828,30 +828,48 @@ Returns:
 */
 string innerEvalCode(string[] eval)
 {
+    import std.conv : text;
     import std.string : join, stripRight;
     // assumeSafeAppend just to avoid unnecessary reallocation
     string code = eval.join("\n").stripRight.assumeSafeAppend;
-    if (code.length > 0 && code[$ - 1] != ';')
-        code ~= ';';
+    size_t lastSemicolon;
+    if (code.empty || code[$ - 1] == ';')
+        goto Lret;
+    // TODO: phobos should have findSplit!(reverse.Flag = reverse.No);
+    lastSemicolon = code.lastIndexOf("@");
+    if(lastSemicolon==-1)
+        goto Lret;  // probably a user error
+    code = text(code[0..lastSemicolon], "writeln(", code[lastSemicolon+1..$], ");");
+    Lret:
     return code;
 }
 
 unittest
 {
-    assert(innerEvalCode([`writeln("Hello!")`]) == `writeln("Hello!");`);
+    assert(innerEvalCode([`writeln("Hello!")`]) == `writeln("Hello!")`);
     assert(innerEvalCode([`writeln("Hello!");`]) == `writeln("Hello!");`);
 
     // test with trailing whitespace
-    assert(innerEvalCode([`writeln("Hello!")  `]) == `writeln("Hello!");`);
+    assert(innerEvalCode([`writeln("Hello!")  `]) == `writeln("Hello!")`);
     assert(innerEvalCode([`writeln("Hello!");  `]) == `writeln("Hello!");`);
 
     // test with multiple entries
     assert(innerEvalCode([`writeln("Hello!");  `, `writeln("You!")  `])
-           == "writeln(\"Hello!\");  \nwriteln(\"You!\");");
+           == "writeln(\"Hello!\");  \nwriteln(\"You!\")");
     assert(innerEvalCode([`writeln("Hello!");  `, `writeln("You!"); `])
            == "writeln(\"Hello!\");  \nwriteln(\"You!\");");
 }
 
+unittest
+{
+    assert(innerEvalCode(["@2"]) == "writeln(2);");
+    assert(innerEvalCode(["@2 + 2"]) == "writeln(2 + 2);");
+    assert(innerEvalCode(["@2 + 2; "]) == "@2 + 2;");
+    assert(innerEvalCode(["2 + 2; @2 + 2"]) == "2 + 2; writeln(2 + 2);");
+    assert(innerEvalCode(["@2.pow(4)"]) == "writeln(2.pow(4));");
+    assert(innerEvalCode(["if(0) {} @2 + 2"]) == "if(0) {} writeln(2 + 2);");
+}
+
 /**
 Formats the code provided via `--eval` or `--loop` flags into a
 string of complete program code that can be written to a file
@@ -891,7 +909,7 @@ unittest
     // innerEvalCode already tests the cases for different
     // contents in `eval` array, so let's focus on testing
     // the difference based on the `loop` flag
-    assert(makeEvalCode([`writeln("Hello!") `], No.loop) ==
+    assert(makeEvalCode([`writeln("Hello!"); `], No.loop) ==
            importWorld
            ~ "void main(char[][] args) {\n"
            ~ "writeln(\"Hello!\");\n}");

rdmd_test.d

@@ -183,6 +183,19 @@ void runTests(string rdmdApp, string compiler, string model)
     assert(res.status == 0, res.output);
     assert(res.output.canFind("eval_works"));  // there could be a "DMD v2.xxx header in the output"
 
+    // Test automatic .writeln for --eval
+    import std.conv : text;
+    import std.typecons : tuple;
+    foreach (t; [tuple(`@"eval_works"`, "eval_works"),
+                 tuple("@2 + 2", "4"),
+                 tuple("2.write; @2 + 2", "24")])
+    {
+        res = execute(rdmdArgs ~ ["--force", "-de", text("--eval=", t[0])]);
+        assert(res.status == 0, res.output);
+        // there could be a "DMD v2.xxx header in the output" (NB: only seems to be the case for GDC)
+        assert(res.output.canFind(t[1]), text("got:", res.output, " expected:", t[1]));
+    }
+
     // compiler flags
     res = execute(rdmdArgs ~ ["--force", "-debug",
         "--eval=debug {} else assert(false);"]);
@@ -256,22 +269,24 @@ void runTests(string rdmdApp, string compiler, string model)
     {
     auto testLines = "foo\nbar\ndoo".split("\n");
 
-    auto pipes = pipeProcess(rdmdArgs ~ ["--force", "--loop=writeln(line);"], Redirect.stdin | Redirect.stdout);
-    foreach (input; testLines)
-        pipes.stdin.writeln(input);
-    pipes.stdin.close();
-
-    while (!testLines.empty)
+    // Test --loop with automatic writeln
+    foreach (loopArg; ["--loop=writeln(line);", "--loop=@line"])
     {
-        auto line = pipes.stdout.readln.strip;
-        if (line.empty || line.startsWith("DMD v")) continue;  // git-head header
-        assert(line == testLines.front, "Expected %s, got %s".format(testLines.front, line));
-        testLines.popFront;
-    }
-    auto status = pipes.pid.wait();
-    assert(status == 0);
-    }
+        auto pipes = pipeProcess(rdmdArgs ~ ["--force", loopArg], Redirect.stdin | Redirect.stdout);
+        foreach (input; testLines)
+            pipes.stdin.writeln(input);
+        pipes.stdin.close();
 
+        while (!testLines.empty)
+        {
+            auto line = pipes.stdout.readln.strip;
+            if (line.empty || line.startsWith("DMD v")) continue;  // git-head header
+            assert(line == testLines.front, "Expected %s, got %s".format(testLines.front, line));
+            testLines.popFront;
+        }
+        auto status = pipes.pid.wait();
+        assert(status == 0);
+    }}
     // vs program file
     res = execute(rdmdArgs ~ ["--force",
         "--loop=assert(true);", voidMain]);