proposal/custom-type-errors.md

@@ -164,4 +164,25 @@ instanceTypeError"You can't compare functions for equality"=>Eq(a -> b)where(==)
 
 1. Do we support `foo :: TypeError (Text "") -> TypeError (Text ""); foo = id`? I don't have a strong feeling one way or the other, but it would be nice to have this specified.  *Lennart*: I would expect this to result in a type error at some point.  If, after typechecking, there are any residual `TypeError` occurrences in types then something has gone wrong.
 
+#### Iavor's Answers to RAE's Questions
+
+
+I think of `TypeErrror` as a rather special type function, which only makes sense in two places in the syntax of the language:
+
+1. as the only constraint in the context of an instance declaration, and
+1. as the right-hand side of a type family instance.
+
+
+The idea is that whenever a type function is about to reduce to `TypeError`, we stop the evaluation and record a custom type error to be reported by GHC.  It doesn't matter if the function evaluation was in the context of a given or wanted constraint.  Similarly, if an instance ever reduces to `TypeError`, then we also report a custom type error.
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+
+With this in mind the answers to your questions would be:
+
+1. `TypeError` should not appear directly in a given context. 
+1. As mentioned above, the methods of an instance with `TypeError` in the context are irrelevant.  I think it is OK to simply leave them unspecified, or define them as `undefined`.
+1. `TypeError` should not appear in types written by the user, except for the two special cases above.  So no `TypeError` in user-defined type signatures, type declarations, class instance heads, parameters to type functions, etc.
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+
+The current implementation does not have any explicit checks to make sure that programmers don't write `TypeError` in places where they are not supposed to. It might be nice to implement such a check, but I am not sure it is really necessary.   With the current implementation, if you wrote `TypeError` in a place that you were not supposed to, you would get the same behavior as for any other type function with no instances.
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