Refuse to derive NoThunks for single-constructor-single-field types #25
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The default implementation of `wNoThunks` translates its argument to a generic representation, _then forces that generic representation to WHNF_, and then calls `gwNoThunks`. This is wrong; given a datatype with a single constructor with a single field ``` data T = MkT Int ``` the generic representation of `MkT x` is `M1 (M1 (K1 x))`, where `M1` and `K1` are both newtypes. This means that forcing the representation to WHNF means we would force the `Int` to WHNF, which is incorrect: calling `noThunks` should never force any values to WHNF that aren't already. Admittedly, this is an edge case, but I think the forcing to WHNF is simply not required: the implementation of `gwNoThunks` does not rely on it. This commit does not fix the problem, only adds the test case that illustrates it.
This doesn't change anything fundamentally, but it avoids giving the impression that that forcing actually does anything useful, which it does not.
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It would be good if this could be checked against a larger code, just to verify that I didn't miss any instances for |
edsko
changed the title
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@edsko have you ever tested this against a larger code base? |
No, but you are in the perfect place to do so 😬 |
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See detailed discussion both in #24 and inline in the code in the discussion of
GCheckEdgeCase. After this PR, trying to defineNoThunksfor a single-constructor-single-field type such aswill result in a custom error message like this:
It's somewhat frustrating that we cannot do better, but this is what we have for now (unless we switch to TH deriving instead of using generics).
Closes #24.