Implemented a file parser. Added SourceOfExamples for lazy example listing (especially useful when lazily searching for files that match a certain prefix).

util/complete/src/main/scala/sbt/complete/Parser.scala

@@ -87,6 +87,9 @@ sealed trait RichParser[A]
 	/** Explicitly defines the completions for the original Parser.*/
 	def examples(s: Set[String], check: Boolean = false): Parser[A]
 
+	/** Explicitly defines the completions for the original Parser.*/
+	def examples(s: SourceOfExamples, maxNumberOfExamples: Int): Parser[A]
+
 	/** Converts a Parser returning a Char sequence to a Parser returning a String.*/
 	def string(implicit ev: A <:< Seq[Char]): Parser[String]
 
@@ -285,6 +288,7 @@ trait ParserMain
 		def - (o: Parser[_]) = sub(a, o)
 		def examples(s: String*): Parser[A] = examples(s.toSet)
 		def examples(s: Set[String], check: Boolean = false): Parser[A] = Parser.examples(a, s, check)
+		def examples(s: SourceOfExamples, maxNumberOfExamples: Int): Parser[A] = Parser.examples(a, s, maxNumberOfExamples)
 		def filter(f: A => Boolean, msg: String => String): Parser[A] = filterParser(a, f, "", msg)
 		def string(implicit ev: A <:< Seq[Char]): Parser[String] = map(_.mkString)
 		def flatMap[B](f: A => Parser[B]) = bindParser(a, f)
@@ -430,6 +434,17 @@ trait ParserMain
 		}
 		else a
 
+	def examples[A](a: Parser[A], completions: SourceOfExamples, maxNumberOfExamples: Int): Parser[A] =
+		if(a.valid) {
+			a.result match
+			{
+				case Some(av) => success( av )
+				case None =>
+					new DynamicExamples(a, completions, maxNumberOfExamples)
+			}
+		}
+		else a
+
 	def matched(t: Parser[_], seen: Vector[Char] = Vector.empty, partial: Boolean = false): Parser[String] =
 		t match
 		{
@@ -703,6 +718,26 @@ private final class Examples[T](delegate: Parser[T], fixed: Set[String]) extends
 			Completions(fixed map(f => Completion.suggestion(f)) )
 	override def toString = "examples(" + delegate + ", " + fixed.take(2) + ")"
 }
+abstract class SourceOfExamples
+{
+	def apply(): Iterable[String]
+	def withAddedPrefix(addedPrefix: String): SourceOfExamples
+}
+private final class DynamicExamples[T](delegate: Parser[T], sourceOfExamples: SourceOfExamples, maxNumberOfExamples: Int = 10) extends ValidParser[T]
+{
+	def derive(c: Char) = examples(delegate derive c, sourceOfExamples.withAddedPrefix(c.toString), maxNumberOfExamples)
+	def result = delegate.result
+	lazy val resultEmpty = delegate.resultEmpty
+	def completions(level: Int) = {
+		if(sourceOfExamples().isEmpty)
+			if(resultEmpty.isValid) Completions.nil else Completions.empty
+		else {
+			val examplesBasedOnTheResult = sourceOfExamples().take(maxNumberOfExamples).toSet
+			Completions(examplesBasedOnTheResult.map(ex => Completion.suggestion(ex)))
+		}
+	}
+	override def toString = "examples(" + delegate + ", " + sourceOfExamples().take(2).toList + ")"
+}
 private final class StringLiteral(str: String, start: Int) extends ValidParser[String]
 {
 	assert(0 <= start && start < str.length)

util/complete/src/main/scala/sbt/complete/Parsers.scala

@@ -7,6 +7,7 @@ package sbt.complete
 	import java.io.File
 	import java.net.URI
 	import java.lang.Character.{getType, MATH_SYMBOL, OTHER_SYMBOL, DASH_PUNCTUATION, OTHER_PUNCTUATION, MODIFIER_SYMBOL, CURRENCY_SYMBOL}
+    import java.nio.file.{Files, Path}
 
 /** Provides standard implementations of commonly useful [[Parser]]s. */
 trait Parsers
@@ -128,8 +129,32 @@ trait Parsers
 	/** Returns true if `c` is an ASCII letter or digit. */
 	def alphanum(c: Char) = ('a' <= c && c <= 'z') || ('A' <= c && c <= 'Z') || ('0' <= c && c <= '9')
 
-	// TODO: implement
+	class FileExamples(base: Path, prefix: String = "") extends SourceOfExamples {
-	def fileParser(base: File): Parser[File] = token(mapOrFail(NotSpace)(s => new File(s.mkString)), "<file>")
+		private val prefixPath: String = "." + File.separator + prefix
+
+		override def apply(): Iterable[String] = files(base).map(base.relativize).map(_.toString.substring(prefix.length))
+
+		override def withAddedPrefix(addedPrefix: String): FileExamples = new FileExamples(base, prefix + addedPrefix)
+
+		protected def fileStartsWithPrefix(path: Path): Boolean = path.toString.startsWith(prefixPath)
+
+		protected def directoryStartsWithPrefix(path: Path): Boolean = {
+			val pathString = path.toString
+			pathString.startsWith(prefixPath) || prefixPath.startsWith(pathString)
+		}
+
+		protected def files(directory: Path): Iterable[Path] = {
+			import scala.collection.JavaConversions._
+			val subPathStream = Files.newDirectoryStream(directory).toStream
+			val (subDirectories, filesOnly) = subPathStream.partition(path => Files.isDirectory(path))
+			filesOnly.filter(fileStartsWithPrefix) ++ subDirectories.filter(directoryStartsWithPrefix).flatMap(files)
+		}
+	}
+
+	def fileParser(base: File, maxNumberOfExamples: Int = 25): Parser[File] =
+		OptSpace ~> StringBasic
+			.examples(new FileExamples(base.toPath), maxNumberOfExamples)
+			.map(new File(_))
 
 	/** Parses a port number.  Currently, this accepts any integer and presents a tab completion suggestion of `<port>`. */
 	lazy val Port = token(IntBasic, "<port>")