From d4266b53ed53f59ddf37645813bdc53a56344bbc Mon Sep 17 00:00:00 2001
From: Florian Schroedl <flo.schroedl@gmail.com>
Date: Thu, 20 Jan 2022 17:00:00 +0100
Subject: [PATCH] Pretty print errors

---
 src/test.nim | 56 ++++++++++++++++++++++++++++++++++++++++------------
 1 file changed, 43 insertions(+), 13 deletions(-)

diff --git a/src/test.nim b/src/test.nim
index 7b59e01..d8934bb 100644
--- a/src/test.nim
+++ b/src/test.nim
@@ -11,11 +11,6 @@ import print
 {.experimental: "caseStmtMacros".}
 
 type
-  ParseError = tuple
-    unexpected: string
-    expected: seq[string]
-    state: ParserState
-    message: string
 
   ParserState* = ref object
     stream: string
@@ -27,13 +22,25 @@ type
   Parser* = ref object
     state: ParserState
     tokens: seq[Token]
-  ParserResult* = Result[Parser, (Parser, string)]
+
+  ParseErrorKind = enum
+    charMismatchErr
+    endOfStringErr
+  ParserError = ref object
+    kind: ParseErrorKind
+    unexpected: char
+    expected: char
+    index: int
+    parser: Parser
+  ParserResult* = Result[Parser, ParserError]
 
   # Builder[T]* = ref object
   #   parser: ParserResult,
   #   tree: seq[T],
   # BuilderResult* = Result[Builder, (Builder, string)]
 
+# func prettyPrintError*(err: ParserErrror,)
+
 func getOrElse*[T, E](self: Result[T, E], otherwise: T): T =
   if self.isOk():
     self.v
@@ -63,6 +70,18 @@ proc `$`*(x: Parser): string =
   tokens: {indentKey($x.tokens, 2)},
 )"""
 
+proc `$`*(x: ParserError): string =
+  case x:
+    of charMismatchErr(expected: @expected, parser: @parser, index: @index, unexpected: @unexpected):
+      # TODO: Only works for single line right now
+      let original = parser.state.stream
+      let errSpace = " ".repeat(index - 1)
+
+      &"""Parsing Error (Character Mismatch Error):
+{original}
+{errSpace}^ Wanted '{unexpected}' but got '{expected}'"""
+    else: "ParseError"
+
 proc initParser(str: string): ParserResult =
   Parser(
     state: ParserState(
@@ -79,10 +98,14 @@ func ch(c1: char): (Parser -> ParserResult) {.inline.} =
     let newIndex = state.position + 1
 
     if newIndex > (state.stream.len - 1):
-      return err((parser, &"Expected {c1} at {newIndex}, got End of string"))
+      return err(ParserError(
+        kind: endOfStringErr,
+        expected: c1,
+        index: newIndex,
+        parser: parser,
+      ))
     else:
       let c2 = state.stream[newIndex]
-
       if c1 == c2:
         return Parser(
           state: ParserState(
@@ -93,7 +116,13 @@ func ch(c1: char): (Parser -> ParserResult) {.inline.} =
           tokens: parser.tokens & Token(value: c2)
         ).ok()
       else:
-        return err((parser, &"Expected {c2} at {newIndex}, got {c1}"))
+        return err(ParserError(
+          kind: charMismatchErr,
+          unexpected: c2,
+          expected: c1,
+          index: newIndex,
+          parser: parser,
+        ))
 
 func ignore(parserFn: Parser -> ParserResult): (Parser -> ParserResult) {.inline.} =
   ## Parse characters but throw success tokens away
@@ -126,14 +155,14 @@ func str(s: string): (Parser -> ParserResult) {.inline.} =
 
 proc foldTokens[T](
   parserResult: ParserResult,
-  onError: T,
+  onError: ParserError -> T,
   onSuccess: seq[Token] -> T,
 ): T =
   if parserResult.isOk():
     onSuccess(parserResult.unsafeGet().tokens)
   else:
-    onError
-    # onError(parserResult.error)
+    let err = parserResult.error()
+    onError(err)
 
 when isMainModule:
   let fooParser = initParser("FOO_BAR")
@@ -141,11 +170,12 @@ when isMainModule:
     str("FOO"),
     ignore(ch('_')),
     ch('B'),
+    ch('B'),
     ch('A'),
     ch('R'),
   ])
   .foldTokens(
-    "Error",
+    err => $err,
     xs => xs.foldl(a & b.value, "")
   )