Playground to test parsing. Is there simpler way to do this?

ParseTest.swift

import Parsing

/// simple test.
/// First char must exist and must be a char from a to z
/// Followed by zero or more characters from a to z or 0 to 9
/// Must end with a space or be the end of input
///
let validFirstChars = CharacterSet(charactersIn: "a"..."z")
let validFollowingChars = CharacterSet(charactersIn: "0"..."9").union(validFirstChars)

let test = Parse {
	Prefix(minLength: 1, maxLength: 1) { $0.unicodeScalars.isEmpty ?
		false :
		validFirstChars.contains($0.unicodeScalars.first!) }
	Prefix { validFollowingChars.contains($0.unicodeScalars.first!) }
	OneOf { " " ; "" }
}.map { $0.appending($1) }

print("\nstart test\n")
print( "should be 'a123':  ", test.parse("a123")! )
print( "should be 'a123':  ", test.parse("a123 ")! )
print( "should be 'a':  ", test.parse("a")! )
print( "should be 'a':  ", test.parse("a ")! )
print( "should be nil:  " , test.parse("0") )

/* Output should be:

start test

should be 'a123':   a123
should be 'a123':   a123
should be 'a':   a
should be 'a':   a
should be nil:   nil

*/