1 files changed, 85 insertions, 0 deletions
diff --git a/compiler/errorhandling.nim b/compiler/errorhandling.nim
new file mode 100644
index 000000000..2cde9e3fb
--- /dev/null
+++ b/compiler/errorhandling.nim
@@ -0,0 +1,85 @@
+#
+#
+#           The Nim Compiler
+#        (c) Copyright 2021 Andreas Rumpf
+#
+#    See the file "copying.txt", included in this
+#    distribution, for details about the copyright.
+#
+
+## This module contains support code for new-styled error
+## handling via an `nkError` node kind.
+
+import ast, renderer, options, types
+import std/strutils
+
+when defined(nimPreviewSlimSystem):
+  import std/assertions
+
+type
+  ErrorKind* = enum ## expand as you need.
+    RawTypeMismatchError
+    ExpressionCannotBeCalled
+    CustomError
+    WrongNumberOfArguments
+    AmbiguousCall
+
+proc errorSubNode*(n: PNode): PNode =
+  case n.kind
+  of nkEmpty..nkNilLit:
+    result = nil
+  of nkError:
+    result = n
+  else:
+    result = nil
+    for i in 0..<n.len:
+      result = errorSubNode(n[i])
+      if result != nil: break
+
+proc newError*(wrongNode: PNode; k: ErrorKind; args: varargs[PNode]): PNode =
+  assert wrongNode.kind != nkError
+  let innerError = errorSubNode(wrongNode)
+  if innerError != nil:
+    return innerError
+  var idgen = idGeneratorForPackage(-1'i32)
+  result = newNodeIT(nkError, wrongNode.info, newType(tyError, idgen, nil))
+  result.add wrongNode
+  result.add newIntNode(nkIntLit, ord(k))
+  for a in args: result.add a
+
+proc newError*(wrongNode: PNode; msg: string): PNode =
+  assert wrongNode.kind != nkError
+  let innerError = errorSubNode(wrongNode)
+  if innerError != nil:
+    return innerError
+  var idgen = idGeneratorForPackage(-1'i32)
+  result = newNodeIT(nkError, wrongNode.info, newType(tyError, idgen, nil))
+  result.add wrongNode
+  result.add newIntNode(nkIntLit, ord(CustomError))
+  result.add newStrNode(msg, wrongNode.info)
+
+proc errorToString*(config: ConfigRef; n: PNode): string =
+  assert n.kind == nkError
+  assert n.len > 1
+  let wrongNode = n[0]
+  case ErrorKind(n[1].intVal)
+  of RawTypeMismatchError:
+    result = "type mismatch"
+  of ExpressionCannotBeCalled:
+    result = "expression '$1' cannot be called" % wrongNode[0].renderTree
+  of CustomError:
+    result = n[2].strVal
+  of WrongNumberOfArguments:
+    result = "wrong number of arguments"
+  of AmbiguousCall:
+    let a = n[2].sym
+    let b = n[3].sym
+    var args = "("
+    for i in 1..<wrongNode.len:
+      if i > 1: args.add(", ")
+      args.add(typeToString(wrongNode[i].typ))
+    args.add(")")
+    result = "ambiguous call; both $1 and $2 match for: $3" % [
+      getProcHeader(config, a),
+      getProcHeader(config, b),
+      args]