Inconsistent behavior of enumFromTo for bounded enums

[sharkdp]

With the canonical Enum instance for the following data type

data T = A | B | C | D

I get:

> enumFromTo A A :: Array T
[A]
> enumFromTo A B :: Array T
[A,B]
> enumFromTo A C :: Array T
[A,B,C]
> enumFromTo A D :: Array T
[A,B,C]

The last one returns a wrong result with the current implementation for enumFromTo because the unfoldr-function calls succ D, which short-circuits in the Maybe monad and breaks the unfoldr 'loop'.

Note: this only fails for bounded Enums

[garyb]

Good catch.

What do you mean it only fails for bounded enums though? Have you got an example of a non-bounded enum that it wouldn't fail for?

[sharkdp]

What do you mean it only fails for bounded enums though? Have you got an example of a non-bounded enum that it wouldn't fail for?

I believe the error only appears if the upper bound in the enumFromTo lower upper call equals top (alternatively, if succ upper = Nothing). If a data type were truly unbounded, the error would never appear.