Support keyword-only function parameters (#143)

Following a hitherto undocumented feature of Skylark-in-Java,
which in turn follows Python3,
a function declaration may now include optional parameters
after the *args parameter:

```
def f(a, b, c=1, *args, d=2, **kwargs)
```
The parameter d is a "keyword-only" parameter as it can
never by assigned from a positional parameter; all positional
arguments surplus to a, b, and c are put in a tuple and
assigned to args.

To declare a non-variadic function with keyword-only arguments,
the *args parameter is replaced by just *:

```
    def f(a, b, c=1, *, d=2, **kwargs)
```
The * parameter is not a real parameter; it just serves as a
separator between the parameter that may be specified positionally
and the keyword-only ones.

Spec proposal at bazelbuild/starlark#23

Fixes #61

2 files changed, 26 insertions, 14 deletions

diff --git a/internal/compile/compile.go b/internal/compile/compile.go
index 6b43fbe..b313a5c 100644
--- a/internal/compile/compile.go
+++ b/internal/compile/compile.go
@@ -37,7 +37,7 @@ import (
 const debug = false // TODO(adonovan): use a bitmap of options; and regexp to match files
 
 // Increment this to force recompilation of saved bytecode files.
-const Version = 5
+const Version = 6
 
 type Opcode uint8
 
@@ -305,10 +305,11 @@ type Funcode struct {
 	Doc                   string          // docstring of this function
 	Code                  []byte          // the byte code
 	pclinetab             []uint16        // mapping from pc to linenum
-	Locals                []Ident         // for error messages and tracing
+	Locals                []Ident         // locals, parameters first
 	Freevars              []Ident         // for tracing
 	MaxStack              int
 	NumParams             int
+	NumKwonlyParams       int
 	HasVarargs, HasKwargs bool
 }
 
@@ -1705,7 +1706,14 @@ func (fcomp *fcomp) function(pos syntax.Position, name string, f *syntax.Functio
 		fmt.Fprintf(os.Stderr, "resuming %s @ %s\n", fcomp.fn.Name, fcomp.pos)
 	}
 
-	funcode.NumParams = len(f.Params)
+	// def f(a, *, b=1) has only 2 parameters.
+	numParams := len(f.Params)
+	if f.NumKwonlyParams > 0 && !f.HasVarargs {
+		numParams--
+	}
+
+	funcode.NumParams = numParams
+	funcode.NumKwonlyParams = f.NumKwonlyParams
 	funcode.HasVarargs = f.HasVarargs
 	funcode.HasKwargs = f.HasKwargs
 	fcomp.emit1(MAKEFUNC, fcomp.pcomp.functionIndex(funcode))
diff --git a/internal/compile/serial.go b/internal/compile/serial.go
index f6cf807..6056ea7 100644
--- a/internal/compile/serial.go
+++ b/internal/compile/serial.go
@@ -39,6 +39,7 @@ package compile
 //	freevar		[]Ident
 //	maxstack	varint
 //	numparams	varint
+//	numkwonlyparams	varint
 //	hasvarargs	varint (0 or 1)
 //	haskwargs	varint (0 or 1)
 //
@@ -185,6 +186,7 @@ func (e *encoder) function(fn *Funcode) {
 	e.idents(fn.Freevars)
 	e.int(fn.MaxStack)
 	e.int(fn.NumParams)
+	e.int(fn.NumKwonlyParams)
 	e.int(b2i(fn.HasVarargs))
 	e.int(b2i(fn.HasKwargs))
 }
@@ -350,20 +352,22 @@ func (d *decoder) function() *Funcode {
 	freevars := d.idents()
 	maxStack := d.int()
 	numParams := d.int()
+	numKwonlyParams := d.int()
 	hasVarargs := d.int() != 0
 	hasKwargs := d.int() != 0
 	return &Funcode{
 		// Prog is filled in later.
-		Pos:        id.Pos,
-		Name:       id.Name,
-		Doc:        doc,
-		Code:       code,
-		pclinetab:  pclinetab,
-		Locals:     locals,
-		Freevars:   freevars,
-		MaxStack:   maxStack,
-		NumParams:  numParams,
-		HasVarargs: hasVarargs,
-		HasKwargs:  hasKwargs,
+		Pos:             id.Pos,
+		Name:            id.Name,
+		Doc:             doc,
+		Code:            code,
+		pclinetab:       pclinetab,
+		Locals:          locals,
+		Freevars:        freevars,
+		MaxStack:        maxStack,
+		NumParams:       numParams,
+		NumKwonlyParams: numKwonlyParams,
+		HasVarargs:      hasVarargs,
+		HasKwargs:       hasKwargs,
 	}
 }