Provide `Reflect` type class for `Literal` reflection about purescript-literals HOT 7 OPEN

Care to expound?

from purescript-literals.

I'm writting on the paper only (I'm going to compile this later):

class Reflect t (s :: Symbol) where
  reflect :: Proxy (Literal t s) -> t
 
instance reflectNumber :: (Number s) => Reflect Number s where
  reflect _ = Literal.toValue (Literal.Number.numberLit :: Literal Number s)

So we can reflect any Literal at hand and be able to represent default values on the type level just by using Literal n s if I'm not wrong ;-)

from purescript-literals.

I'm not sure if such default values would be usable in practice though... Maybe I should experiment a bit before raising this discussion ;-)

I have doubts because even if we have mechanism to derive a type from defaults it won't be taken into account by the type checker. I mean even if we provide class UnLiteral l t | l -> t and instance unLiteralNumber :: UnLiteral (Literal t s) t we won't be able to type something based on this like:

type MyLiteral = Literal Number "80.0"

operateOnUnliteral :: UnLiteral MyLiteral r' => r' -> o

This doesn't bring the type of r' Number to the function body I think :-(

But maybe there is a context in which such generic reflection could be used.

from purescript-literals.

Possible usage (defaults... and maybe more):

master...paluh:master

from purescript-literals.

@jvliwanag Please take a look at examples - I was able to add an experimental function which based on reflection + typelevel evaluation produces record of fully polymorphic lenses for a given record. Here is an example usage.

P.S.
I should probably use a class from purescript-reflection - I'm going to migrate to type class from there when we agree if we want to introduce reflection instances here (I think it is worth it :-))

P.S.II. I have to revert back this change in Literal kind - there should be a Symbol there.

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@jvliwanag Do we want to include this reflection in the upcoming release? Should I prepare proper PR?

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I'll go try making a pre 1.0 release first. Need to familiarize myself first with the release process. Of course, a PR for your improvements are welcome and we can make another release then.

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