demo: make it cleaner on mobile platforms

Add a flag to the config so the mobile wrapper can report it's on
a mobile platform, and use this to give instructions for )demo on
mobile. Tell the user in that case to use the Demo button, which
works fine on Android but needs to be implemented on iOS,
but it should be easy.

Also refactor the demo runner a bit so that it prints instructions
from demo.ivy before accepting user input.

config/config.go

@@ -49,6 +49,7 @@ type Config struct {
 	// Bases: 0 means C-like, base 10 with 07 for octal and 0xa for hex.
 	inputBase  int
 	outputBase int
+	mobile     bool // Running on a mobile platform.
 }
 
 func (c *Config) init() {
@@ -63,6 +64,7 @@ func (c *Config) init() {
 		c.maxDigits = 1e4
 		c.maxStack = 1e5
 		c.floatPrec = 256
+		c.mobile = false
 	}
 }
 
@@ -312,3 +314,14 @@ func (c *Config) SetBase(inputBase, outputBase int) {
 	c.inputBase = inputBase
 	c.outputBase = outputBase
 }
+
+// Mobile reports whether we are running on a mobile platform.
+func (c *Config) Mobile() bool {
+	return c.mobile
+}
+
+// SetMobile sets the Mobile bit as specified.
+func (c *Config) SetMobile(mobile bool) {
+	c.init()
+	c.mobile = mobile
+}

demo/demo.go

@@ -31,9 +31,18 @@ func Text() string {
 // line from the file is delivered to ivy. If the user's input line has text, that
 // is delivered instead and the file does not advance.
 func Run(userInput io.Reader, toIvy io.Writer, output io.Writer) error {
-	fmt.Println(`# Type a newline.`)
 	text := demoText // Don't overwrite the global!
 	scan := bufio.NewScanner(userInput)
+	nextLine := func() (line []byte) {
+		nl := bytes.IndexByte(text, '\n')
+		if nl < 0 { // EOF or incomplete line.
+			return nil
+		}
+		line, text = text[:nl+1], text[nl+1:]
+		return line
+	}
+	// Show first line, with instructions, before accepting user input.
+	output.Write(nextLine())
 	for scan.Scan() {
 		// User typed something; step back across the newline.
 		if len(scan.Bytes()) > 0 {
@@ -48,15 +57,14 @@ func Run(userInput io.Reader, toIvy io.Writer, output io.Writer) error {
 			}
 		} else {
 			// User typed newline; send next line of file's text.
-			nl := bytes.IndexByte(text, '\n')
-			if nl < 0 { // EOF or incomplete line.
+			line := nextLine()
+			if line == nil {
 				break
 			}
-			output.Write(text[:nl+1]) // Show the line from the file.
-			if _, err := toIvy.Write(text[:nl+1]); err != nil {
+			output.Write(line)
+			if _, err := toIvy.Write(line); err != nil {
 				return err
 			}
-			text = text[nl+1:]
 		}
 	}
 	return scan.Err()

demo/demo.ivy

@@ -1,5 +1,5 @@
-# This is a demo of ivy. Type a newline to advance to each new step.
-# Type the word "quit" or EOF to end the demo and return to ivy.
+# This is a demo of ivy. Type a newline to advance to each new step. Type one now.
+# At any time, type the word "quit" or EOF to end the demo and return to ivy.
 # Each step in the demo is one line of input followed by some output from ivy. Type a newline now to see.
 2+2
 # The first line you see above (2+2) is input; the next (4) is output from a running ivy.

doc.go

@@ -289,7 +289,8 @@ base 10 and must be non-negative on input.
 		Toggle or set the named debugging flag. With no argument, lists
 		the settings.
 	) demo
-		Run a line-by-line interactive demo. Requires a Go installation.
+		Run a line-by-line interactive demo. On mobile platforms,
+		use the Demo menu option instead.
 	) format ""
 		Set the format for printing values. If empty, the output is printed
 		using the output base. If non-empty, the format determines the

mobile/help.go

@@ -340,7 +340,8 @@ base 10 and must be non-negative on input.
 	Toggle or set the named debugging flag. With no argument, lists
 	the settings.
 ) demo
-	Run a line-by-line interactive demo. Requires a Go installation.
+	Run a line-by-line interactive demo. On mobile platforms,
+	use the Demo menu option instead.
 ) format ""
 	Set the format for printing values. If empty, the output is printed
 	using the output base. If non-empty, the format determines the

mobile/mobile.go

@@ -51,6 +51,7 @@ func Eval(expr string) (result string, errors error) {
 
 	conf.SetOutput(stdout)
 	conf.SetErrOutput(stderr)
+	conf.SetMobile(true)
 
 	scanner := scan.New(context, " ", reader)
 	parser := parse.NewParser(" ", scanner, context)

parse/special.go

@@ -185,6 +185,10 @@ Switch:
 		}
 	case "demo":
 		p.need(scan.EOF)
+		if conf.Mobile() {
+			p.Printf("For a demo on mobile platforms, use the Demo button in the UI.\n")
+			break
+		}
 		// Use a default configuration.
 		var conf config.Config
 		err := demo.Run(os.Stdin, p.demoRunner(), conf.Output())