polymorphic record update

[claudeha]

While experimenting with https://github.com/tidalcycles/Tidal and MicroHs I ran into a problem: TidalCycles makes use of polymorphic record update.

https://github.com/augustss/MicroHs?tab=readme-ov-file#records currently says:

Note that record updates cannot change the type of polymorphic fields.

Is this on the roadmap to be fixed in the future?

[augustss]

It's not high on my list. But then, nobody has complained before. :)
I have only needed it once, so I wrote that update function by hand.

It's difficult(impossible?) to do the way record access is done in MicroHs.

[claudeha]

I have a workaround now for the ~7 occurrences of the update, workaround is not 100% satisfactory because missing field updates will leave undefined values in the record which could be a debugging nightmare, instead of being a compile-time type error.

replace

updated = record { field = foo, otherField = bar }

with

polymorphic = fmap (const undefined)  -- the record in question is a Functor
updated = (polymorphic record) { field = foo, otherField = bar }

[augustss]

Here's a safer way to do what you want, assuming the constructor for record is T
updated = let T{..}=record in let field=foo; otherfield=bar in T{..}

BUT, there was a bug in the compiler stopping this from working, so you need to wait for the next version. I'm going to try and fix some more of your bugs for that.

[augustss]

You can use my suggested method in 0.13.0.0, but I'll leave this PR open, since it's something that needs a proper fix.