errors and validation formal definitions

Author: twof

spec/Section 5 -- Validation.md

@@ -429,6 +429,10 @@ SameResponseShape(fieldA, fieldB):
 
   * Let {typeA} be the return type of {fieldA}.
   * Let {typeB} be the return type of {fieldB}.
+  * Let {fieldARequiredStatus} be the required status of {fieldA}
+  * Let {fieldBRequiredStatus} be the required status of {fieldB}
+  * Let {typeA} be the result of {ModifiedOutputType(typeA, fieldARequiredStatus)}
+  * Let {typeB} be the result of {ModifiedOutputType(typeB, fieldBRequiredStatus)}
   * If {typeA} or {typeB} is Non-Null.
     * If {typeA} or {typeB} is nullable, return false.
     * Let {typeA} be the nullable type of {typeA}

spec/Section 6 -- Execution.md

@@ -567,7 +567,7 @@ and a final type is produced.
 
 ModifiedOutputType(outputType, requiredStatus):
   * If {requiredStatus} is 'required' and {outputType} is not a Non-Nullable type:
-    * Return Non-Null with an inner type of {outputType}.
+    * Return `Non-Null` with an inner type of {outputType}.
   * Otherwise if {requiredStatus} is 'optional':
     * If {outputType} is not a Non-Nullable type:
       * Return {outputType}.
@@ -772,9 +772,9 @@ field returned {null}, and the error must be added to the {"errors"} list in
 the response.
 
 If the result of resolving a field is {null} (either because the function to
-resolve the field returned {null} or because a field error was raised), and that
-field is of a `Non-Null` type, then a field error is raised. The
-error must be added to the {"errors"} list in the response.
+resolve the field returned {null} or because a field error was raised), and 
+the {ModifiedOutputType} of that field is of a `Non-Null` type, then a field 
+error is raised. The error must be added to the {"errors"} list in the response.
 
 If the field returns {null} because of a field error which has already been
 added to the {"errors"} list in the response, the {"errors"} list must not be
@@ -783,8 +783,8 @@ field.
 
 Since `Non-Null` type fields cannot be {null}, field errors are propagated to be
 handled by the parent field. If the parent field may be {null} then it resolves
-to {null}, otherwise if it is a `Non-Null` type, the field error is further
-propagated to its parent field.
+to {null}, otherwise if its {ModifiedOutputType} is a `Non-Null` type, the field 
+error is further propagated to its parent field.
 
 If a `List` type wraps a `Non-Null` type, and one of the elements of that list
 resolves to {null}, then the entire list must resolve to {null}.