SuspenseStore refactor to allow caching of any HasBackend instance members

[megamaddu]

The old store required the key and value types of the cache to be defined at cache creation. This refactor is to allow any type implementing HasBackend to be stored in the cache in a (hopefully) type-safe way.

The main bits I'd like feedback on:

• does the HasBackend class behave correctly, i.e. do the functional dependencies correctly prevent invalid instances?
  • an instance is invalid if a given key, k, can be used in multiple instances with a different s or v
  • storing the same entity type using different key types is fine, but it should be impossible to cause overlapping s values across different choices for v.. does that make sense?
  • put another way, s needs to be unique for each choice of v
• the HasBackend implementations are carried into the cache using a few internal classes, ToKey, ToAffUnsafe, and HasOpaque, using Storable -- do these make sense?
• any better ways to accomplish this?

[megamaddu]

Implementing HasBackend and calling the getters above is the external API. The classes and types below should contain the unsafe bits.

[hdgarrood]

I'm not sure this will do what you want it to: with this code, I think I can define instances

HasBackend String Int "1"
HasBackend Int Int "1"

since we are effectively only saying that there can only be one HasBackend instance for each key type k; we aren't preventing you from defining two different instances with different k types but the same s type. A functional dependency s -> k would prevent you from doing this, but that probably isn't workable because you'll fall foul of orphan instance restrictions there - since in this case s determines everything, then the instance can only be defined in the same module as the HasBackend class. Normally you'd also be able to define the instance in the same module as the type s, but in this case s is built in to the compiler so you can't put it there. See https://liamgoodacre.github.io/purescript/type/class/instance/orphan/functional/dependencies/2017/01/22/purescript-orphan-instance-detection.html for more information on how functional dependencies affect orphan instance checking.

I think you'd ideally need something like Haskell's Data.Typeable to do this safely, but PureScript doesn't have that. I'm not sure there's a good way of doing this without Data.Typeable, unfortunately.

[hdgarrood]

Is the type class doing much for us here? In general I think defining a type class and not exporting it is a bit suspect; would normal functions work?

[hdgarrood]

Same goes for HasOpaque

[megamaddu]

Ah, you're right, thanks!

[hdgarrood]

I'm not sure this will do what you want it to: with this code, I think I can define instances

HasBackend String Int "1"
HasBackend Int Int "1"

since we are effectively only saying that there can only be one HasBackend instance for each key type k; we aren't preventing you from defining two different instances with different k types but the same s type. A functional dependency s -> k would prevent you from doing this, but that probably isn't workable because you'll fall foul of orphan instance restrictions there - since in this case s determines everything, then the instance can only be defined in the same module as the HasBackend class. Normally you'd also be able to define the instance in the same module as the type s, but in this case s is built in to the compiler so you can't put it there. See https://liamgoodacre.github.io/purescript/type/class/instance/orphan/functional/dependencies/2017/01/22/purescript-orphan-instance-detection.html for more information on how functional dependencies affect orphan instance checking.

I think you'd ideally need something like Haskell's Data.Typeable to do this safely, but PureScript doesn't have that. I'm not sure there's a good way of doing this without Data.Typeable, unfortunately.

[i-am-the-slime]

The suspense on this one is killing me!

[megamaddu]

@i-am-the-slime tbh I'm not sure when I'll get to this. My new day job isn't using PS and I'm a bit burnt out on the after hours programming. Maybe someday inspiration will strike, but in the meantime I'll do my best to review and merge any work others want to contribute to this.

[i-am-the-slime]

@spicydonuts
It's quite sad that your new job does not use PS. All good, please take all the time you need for yourself and only do this when you really feel like it! Your physical and mental health(s?) will thank you for it.