Add optional parser method

src/parser/parser.nim

@@ -203,6 +203,16 @@ func ignore*(parserFn: Parser -> ParserResult): (Parser -> ParserResult) {.inlin
       tokens: parser.tokens,
     ))
 
+func optional*(parserFn: Parser -> ParserResult): (Parser -> ParserResult) {.inline.} =
+  ## Parse characters and ignore failure
+  return proc(parser: Parser): ParserResult =
+    let newParser = parserFn(parser)
+
+    if newParser.isOk():
+      newParser
+    else:
+      parser.ok()
+
 func manyUntil*(acceptFn: Parser -> ParserResult, stopFn: Parser -> ParserResult): (Parser -> ParserResult) {.inline.} =
   ## Parse characters but throw success tokens away
   return proc(parser: Parser): ParserResult =
@@ -241,6 +251,7 @@ func choice*(parsers: seq[Parser -> ParserResult]): (Parser -> ParserResult) {.i
       proc(x: ParserResult): ParserResult = x,
     )
 
+
 proc parseSeq*(parser: ParserResult, xs: seq[Parser -> ParserResult]): ParserResult =
   xs.foldl(a.flatMap(b), parser)
 
@@ -307,35 +318,10 @@ proc foldBuilder*[T, T2](
     onError(err[1])
 
 when isMainModule:
-  let parseHeadingStars = @[
-    manyUntil(ch('*'), ch(' ')),
-    ignore(ch(' '))
+  let optionalPrefixParser = @[
+    optional(ch('_')),
+    str("ABC")
   ]
 
-  let parseHeadingText = @[
-    anyUntil(endOfStream),
-  ]
-
-  type StringBuilderT = string
-  type StringBuilder = Builder[StringBuilderT]
-  type StringBuilderResult = BuilderResult[StringBuilderT]
-  proc stringConcat(typeInfo: StringBuilderT):
-                   (seq[Token], seq[StringBuilderT]) -> seq[StringBuilderT] =
-    return proc(xs: seq[Token], ys: seq[StringBuilderT]): seq[StringBuilderT] =
-      return ys & xs.foldl(a & b.value, typeInfo)
-
-  let sampleBuilder = StringBuilderResult
-  .ok(StringBuilder((
-    parser: initParser("**** Some stars"),
-    tree: newSeq[StringBuilderT](),
-  )))
-  .applyParsersSeq(@[
-    (parseHeadingStars, stringConcat("Stars: ")),
-    (parseHeadingText, stringConcat("Text: "))
-  ])
-  .foldBuilder(
-    err => &"Error Parsing: {err}",
-    xs => "Parser Succesfull:\n" & xs.join("\n"),
-  )
-
-  echo sampleBuilder
+  echo initParserResult("_ABC").parseSeq(optionalPrefixParser)
+  echo initParserResult("ABC").parseSeq(optionalPrefixParser)