Add Homogeneous and HomogeneousRowList for homogeneous rows

[paulyoung]

This is my first attempt at type-level programming. Hopefully it's pretty self explanatory, but the idea is to make sure that every field in a record has the same type.

Something similar can be achieved using StrMap, but it's not quite the same.

This initial version is available to play with at http://try.purescript.org/?gist=bec330aab7a6c0b21e466e37483dfba9. It was originally adapted from http://try.purescript.org/?gist=2720825b0476c8924717437fb6f6eefb.

I think it would be great to support polymorphism and open rows, but I haven't had any success with that.

Any and all feedback is welcome.

[paf31]

Looks good, thanks! A few notes though:

• I'd rather call this something like Homogeneous, and have it be a predicate on the row type, not the record type.
• It should use TypeEquals, to force the types to be equal by unification.
• I think you can add a fundep from the row to the type in the labels, which might be useful.

[LiamGoodacre]

@paf31 is the motivation for the TypeEquals approach, that you get a nicer error and don't see the RowList implementation artifact?

[paf31]

Yes, since the type is already determined if the fundep is added anyway.

[paulyoung]

Thanks for the feedback so far. Good call on moving this to Row.

Here's what I have so far:

class Homogeneous row fieldType
instance homogeneous
  :: ( RowToList row fields
     , FieldOf fields fieldType
     )
  => Homogeneous (? row) fieldType -- not sure what to do here

class FieldOf (rowList :: RowList) fieldType
instance fieldOfCons
  :: (FieldOf tail fieldType)
  => FieldOf (Cons symbol fieldType tail) fieldType
instance fieldOfNil :: FieldOf Nil fieldType

Does this seems like I'm on the right track?

I'm not quite sure about the other 2 bullet points but eager to understand.

[LiamGoodacre]

@paulyoung The Homogeneous type class will need a kind annotation of # Type on the row parameter. For the instance, all the arguments can be left as type variables. I.e:

instance homogeneous
  :: ( RowToList row fields
     , FieldOf fields fieldType
     )
  => Homogeneous row fieldType

Phil's point about TypeEquals is referring to the FieldOf (Cons symbol fieldType tail) fieldType instance. The fieldType appears twice across multiple instance arguments. This isn't necessarily bad, it's just that if there is a mismatch the error will mention that it couldn't find an instance for FieldOf .... We'd rather have an error that two types weren't equal. So the action here is to rename one of the two usages of fieldType and add a new instance constraint requiring that those two types be equal.

The functional dependencies for FieldOf should be | rowList -> fieldType. Because once we know which instance to select - which we do by looking at the rowList, i.e whether or not it is Nil or Cons - and the constraints have been satisfied, the fieldType should be fully determined. The same sort of functional dependencies could be used on Homogeneous.

For future reading, the following is incorrect. If you require multiple constraints with the same type class they will conflict and give you overlapping instances.

In cases where there is only one instance with all the arguments being type variables, I usually have both parameters always determined | -> row fieldType - but I think Phil sometimes disagrees with doing that? @paf31

[paf31]

I don't think we can have both parameters determined, since that says you can predict the row given no information.

[paulyoung]

Thanks for the helpful feedback so far. I pushed some changes.

[paulyoung]

Is there anything else I can do here?

[paf31]

@LiamGoodacre Any thoughts on this? What do you think about creating some submodules here?

[LiamGoodacre]

Just been thinking about the fundeps and I'm thinking that this might suffer from the same issue I mentioned about multiple constraints. I'll try to come up with a demonstration tonight.

[LiamGoodacre]

Actually, I am mistaken. Ignore my previous concern.

[LiamGoodacre]

Submodules sound good. What do you suggest here? @paf31

[paulyoung]

Should I put this in a Type.Row.Homogeneous module?

[paf31]

Should I put this in a Type.Row.Homogeneous module?

Sorry for the delay. Yes please.

[paulyoung]

I think I've addressed everything in the latest round of feedback.

[paf31]

Thanks!