Add fold fn for builder

src/test.nim

@@ -302,6 +302,17 @@ proc applyParsersSeq[T](
   ]]): BuilderResult[T] =
     xs.foldl(a.flatMap((x: Builder[T]) => x.applyParsers(b[0], b[1])), builder)
 
+proc foldBuilder[T, T2](
+  builderResult: BuilderResult[T],
+  onError: string -> T2,
+  onSuccess: seq[T] -> T2,
+): T =
+  if builderResult.isOk():
+    onSuccess(builderResult.unsafeGet().tree)
+  else:
+    let err = builderResult.error()
+    onError(err[1])
+
 when isMainModule:
   let parseHeadingStars = @[
     manyUntil(ch('*'), ch(' ')),
@@ -329,15 +340,9 @@ when isMainModule:
     (parseHeadingStars, stringConcat("Stars: ")),
     (parseHeadingText, stringConcat("Text: "))
   ])
-
-  # .flatMap((x: StringBuilder) => x.applyParsers(
-  #   parsers = parseHeadingStars,
-  #   tokenFoldFn = stringConcat("Stars: ")
-  # ))
-  # .flatMap((x: StringBuilder) => x.applyParsers(
-  #   parsers = parseHeadingText,
-  #   tokenFoldFn = stringConcat("Text: ")
-  # ))
-
+  .foldBuilder(
+    err => &"Error Parsing: {err}",
+    xs => "Parser Succesfull:\n" & xs.join("\n"),
+  )
 
   echo sampleBuilder