diff -r 955d3add9426 -r 4b3436c03726 errors.go
--- /dev/null	Thu Jan 01 00:00:00 1970 +0000
+++ b/errors.go	Sun Oct 16 11:52:01 2016 +0200
@@ -0,0 +1,63 @@
+package takuzu
+
+import "fmt"
+
+// This file contains the takuzu validation error type.
+
+const (
+	ErrorNil = iota
+	ErrorDuplicate
+	ErrorTooManyValues
+	ErrorTooManyAdjacentValues
+)
+
+type validationError struct {
+	ErrorType    int
+	LineNumber   *int
+	ColumnNumber *int
+	CellValue    *int
+}
+
+func (e validationError) Error() string {
+	var axis string
+	var n int
+
+	// Currently we don't have validation errors with both
+	// line and column so we can get the axis:
+	if e.LineNumber != nil {
+		axis = "line"
+		n = *e.LineNumber
+	} else if e.ColumnNumber != nil {
+		axis = "column"
+		n = *e.ColumnNumber
+	}
+
+	switch e.ErrorType {
+	case ErrorNil:
+		return ""
+	case ErrorDuplicate:
+		if axis == "" {
+			return "internal validation error"
+		}
+		return fmt.Sprintf("duplicate %ss (%d)", axis, n)
+	case ErrorTooManyValues:
+		if axis == "" || e.CellValue == nil {
+			return "internal validation error"
+		}
+		var numberStr string
+		if *e.CellValue == 0 {
+			numberStr = "zeroes"
+		} else if *e.CellValue == 1 {
+			numberStr = "ones"
+		} else {
+			return "internal validation error"
+		}
+		return fmt.Sprintf("%s %d: too many %s", axis, n, numberStr)
+	case ErrorTooManyAdjacentValues:
+		if axis == "" || e.CellValue == nil {
+			return "internal validation error"
+		}
+		return fmt.Sprintf("%s %d: 3+ same values %d", axis, n, *e.CellValue)
+	}
+	return "internal validation error"
+}