clash-compiler
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clash-lang
Expose `Clash.Class.Counter` methods
Currently, functions like countMin and countSuccOverflow are marked in the documentation as internal implementation details that shouldn't be used by client code. But they (or something similar) would be very useful in a lot of cases where we only want to count "once" and then do something else.
I realize overflow detection can be achieved "on the outside" by adding one more bit to the counter, without access to these functions by doing something like:
let ((bitCoerce @(Index 2) -> overflow), x') = countSucc (0, x)
in x' <$ guard (not overflow)
but this is not really readable (it could be improved a bit if we had a Counter Bool instance). And I'm not sure what similar trick could apply to countMin.
I think the API I would prefer is one that consists of countMin, and countSuccChecked :: (Counter a) => a -> Maybe a. Then both countSucc and countSuccOverflow are trivially recoverable:
countSucc = fromMaybe countMin . countSuccChecked
countSuccOverflow = maybe (True, countMin) (False,) . countSuccChecked
The nice thing about (Bool, a) is that you don't need a branch to determine the overflowed number. While this is a bit of a micro-optimization, I do feel it's the right thing to do.
How about simply moving the internal functions public? I don't think we discovered any downsides to the private API so far, so as far as I'm concerned it's fine as it is.
Would simply exposing the internal functions be okay? In that case, it's easily done.
We could also add an instance for Counter a => Maybe a
And then you can get your interface with:
countSuccChecked :: Counter a => a -> Maybe a
countSuccChecked = countSucc . Just
@gergoerdi I'd be happy to move this forward; could you comment on my replies?
I'm happy with the suggested ways of doing thidls, because the Maybe instance would give me what I need.