Implement choice parser

src/test.nim

@@ -24,6 +24,7 @@ type
     tokens: seq[Token]
 
   ParseErrorKind = enum
+    choiceMismatchErr
     charMismatchErr
     endOfStringErr
   ParserError = ref object
@@ -96,6 +97,13 @@ proc `$`*(x: ParserError): string =
 
       &"""Parsing Error (Character Mismatch Error):
 {original}
+{errSpace}^ Expected '{expected}' but got '{unexpected}'"""
+    of choiceMismatchErr(expected: @expected, parser: @parser, index: @index, unexpected: @unexpected):
+      let original = parser.state.stream
+      let errSpace = " ".repeat(max(0, index))
+
+      &"""Parsing Error (Character Mismatch Error):
+{original}
 {errSpace}^ Expected '{expected}' but got '{unexpected}'"""
     else: "ParseError"
 
@@ -204,6 +212,33 @@ func manyUntil(acceptFn: Parser -> ParserResult, stopFn: Parser -> ParserResult)
 func anyUntil(stopFn: Parser -> ParserResult): (Parser -> ParserResult) {.inline.} =
   manyUntil(ch(AllChars), stopFn)
 
+func choice(parsers: seq[Parser -> ParserResult]): (Parser -> ParserResult) {.inline.} =
+  return proc(parser: Parser): ParserResult =
+    var errors: seq[ParserResult] = newSeq[ParserResult]()
+    var found = Nothing[ParserResult]()
+
+    for fn in parsers:
+      let fnResult: ParserResult = fn(parser)
+
+      if fnResult.isOk():
+        found = fnResult.just
+        break
+      else:
+        errors = errors & fnResult
+
+    return found
+    .fold(
+      proc(): ParserResult =
+        let prettyErrors = errors.map((x: ParserResult) => x.error().expected)
+        err(ParserError(
+          kind: choiceMismatchErr,
+          expected: &"Choice ({prettyErrors})",
+          unexpected: errors[0].error().unexpected,
+          parser: parser,
+        )),
+      proc(x: ParserResult): ParserResult = x,
+    )
+
 proc parseSeq(parser: ParserResult, xs: seq[Parser -> ParserResult]): ParserResult =
   xs.foldl(a.flatMap(b), parser)
 
@@ -240,10 +275,8 @@ when isMainModule:
   #   xs => xs.foldl(a & b.value, "")
   # )
 
-
-
-  echo initParser("_FOO___BAR").parseSeq(@[
-    anyUntil(endOfStream),
+  echo initParser("BB").parseSeq(@[
+    choice(@[ch(Digits), ch('B')]),
   ])