add 'printf' and 'sprintf' builtin functions

docs/builtins.md

@@ -7,6 +7,29 @@ Prints a string representation of the given variable to the standard output.
 ```golang
 v := [1, 2, 3]
 print(v)  // "[1, 2, 3]"
+
+print(1, 2, 3)
+// "1"
+// "2"
+// "3"
+```
+
+## printf
+
+Prints a formatted string to the standard output. It does not append the newline character at the end. The first argument must a String object. It's same as Go's `fmt.Printf`.
+
+```golang
+a := [1, 2, 3]
+printf("foo %v", a)  // "foo [1, 2, 3]"
+```
+
+## sprintf
+
+Returns a formatted string. The first argument must be a String object. It's the same as Go's `fmt.Sprintf`.
+
+```golang
+a := [1, 2, 3]
+b := sprintp("foo %v", a)  // b == "foo [1, 2, 3]" 
 ```
 
 ## len

objects/builtin_print.go

@@ -4,6 +4,7 @@ import (
 	"fmt"
 )
 
+// print(args...)
 func builtinPrint(args ...Object) (Object, error) {
 	for _, arg := range args {
 		if str, ok := arg.(*String); ok {
@@ -15,3 +16,52 @@ func builtinPrint(args ...Object) (Object, error) {
 
 	return nil, nil
 }
+
+// printf("format", args...)
+func builtinPrintf(args ...Object) (Object, error) {
+	numArgs := len(args)
+	if numArgs == 0 {
+		return nil, ErrWrongNumArguments
+	}
+
+	format, ok := args[0].(*String)
+	if !ok {
+		return nil, ErrInvalidTypeConversion
+	}
+	if numArgs == 1 {
+		fmt.Print(format)
+		return nil, nil
+	}
+
+	formatArgs := make([]interface{}, numArgs-1, numArgs-1)
+	for idx, arg := range args[1:] {
+		formatArgs[idx] = objectToInterface(arg)
+	}
+
+	fmt.Printf(format.Value, formatArgs...)
+
+	return nil, nil
+}
+
+// sprintf("format", args...)
+func builtinSprintf(args ...Object) (Object, error) {
+	numArgs := len(args)
+	if numArgs == 0 {
+		return nil, ErrWrongNumArguments
+	}
+
+	format, ok := args[0].(*String)
+	if !ok {
+		return nil, ErrInvalidTypeConversion
+	}
+	if numArgs == 1 {
+		return format, nil // okay to return 'format' directly as String is immutable
+	}
+
+	formatArgs := make([]interface{}, numArgs-1, numArgs-1)
+	for idx, arg := range args[1:] {
+		formatArgs[idx] = objectToInterface(arg)
+	}
+
+	return &String{Value: fmt.Sprintf(format.Value, formatArgs...)}, nil
+}

objects/builtins.go

@@ -12,6 +12,14 @@ var Builtins = []NamedBuiltinFunc{
 		Name: "print",
 		Func: builtinPrint,
 	},
+	{
+		Name: "printf",
+		Func: builtinPrintf,
+	},
+	{
+		Name: "sprintf",
+		Func: builtinSprintf,
+	},
 	{
 		Name: "len",
 		Func: builtinLen,

runtime/vm_builtin_test.go

@@ -140,4 +140,13 @@ func TestBuiltinFunction(t *testing.T) {
 
 	expect(t, `out = from_json("5")`, 5.0)
 	expect(t, `out = from_json("[\"bar\",1,1.8,56,true]")`, ARR{"bar", 1.0, 1.8, 56.0, true})
+
+	// sprintf
+	expect(t, `out = sprintf("")`, "")
+	expect(t, `out = sprintf("foo")`, "foo")
+	expect(t, `out = sprintf("foo %d %v %s", 1, 2, "bar")`, "foo 1 2 bar")
+	expect(t, `out = sprintf("foo %v", [1, "bar", true])`, "foo [1 bar true]")
+	expect(t, `out = sprintf("foo %v %d", [1, "bar", true], 19)`, "foo [1 bar true] 19")
+	expectError(t, `sprintf(1)`)   // format has to be String
+	expectError(t, `sprintf('c')`) // format has to be String
 }