Partial query evaluation

[garyb]

Check it out @natefaubion!

Two-fold rationale:

1. we looked into it before of course, but I couldn't figure out the plumbing - so the same reasons as then, allows proxy components to fail properly, etc.
2. I wanted to make a less boilerplatey construction method for the components again, and by allowing query eval to return Nothing that leaves us free to have the query type polymorphic until a real eval is provided for it.

I've only updated two of the examples, for proof that it's usable, as I wanted to see if we can think of reasons why we shouldn't do this.

[garyb]

I guess these could be Maybe (HalogenM s act ps o m Unit) rather than going via action handling, but I kinda like handling these in one place (which is already true if HalogenQ is handled rather than going the mkEval route).

[natefaubion]

I don't think they would need to be Maybe, since they can just be pure unit.

[garyb]

That's true - it was more a comment on them mapping to act rather than operating directly in HalogenM.

[natefaubion]

I think this looks good. Since we have Request, maybe Handle should be Action?

[natefaubion]

I think this looks good. Since we have Request, maybe Handle should be Action?

[safareli]

What you think about calling mkEval from mkComponent (i.e. ComponentSpec taking EvalSpec ?

[garyb]

We can't do that without exposing another different version of mkComponent, since Halogen uses it internally. Plus I wanted to provide the choice of writing an eval in terms of HalogenQ directly.

Having mkEval in there seemed better given both of those considerations, as it should avoid some confusion about things like when we had 4 different component constructors before - they were all the same aside from some synonyms and field-defaulting, but that wasn't obvious to the casual user. Since the same actual component constructor is used here, and we just use another function to build up part of its internals, it shows that there's nothing special going on, no matter how the component is constructed.

[acple]

more I like mkEval has this type:

mkEval
  :: forall s f act ps i o m
   . (EvalSpec s f act ps i o m -> EvalSpec s f act ps i o m)
  -> HalogenQ f act i ~> HalogenM s act ps o m

does no longer require expose defaultEval.
eval: H.mkEval _{ handleAction = handleAction }

[garyb]

I did consider that too, or exporting a function that does that as well as having a defaultEval record.