Add Homogeneous and HomogeneousRowList for homogeneous rows #20

paulyoung · 2017-08-10T07:15:20Z

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.

paulyoung · 2017-08-10T07:19:20Z

src/Type/Data/Record.purs

+  :: ( RowToList record fields
+     , FieldOf fields fieldType
+     )
+  => RecordOf (Record record) fieldType


I think (Record record) is the right thing to do here (as opposed to just record) but it has the consequence of changing the type of a function beyond simply adding the type constraint.

For example, this:

foo :: forall a . Record a -> Record a

becomes:

foo :: forall a . RecordOf a Boolean => a -> a

paf31 · 2017-08-10T16:29:27Z

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 · 2017-08-10T18:55:17Z

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

paf31 · 2017-08-10T19:51:32Z

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

paulyoung · 2017-08-11T04:51:05Z

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 · 2017-08-11T08:02:43Z

@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 · 2017-08-11T16:58:02Z

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

* Add `TypeEquals` constraint on field type * Add funcitonal dependency for `FieldOf`

paulyoung · 2017-08-24T05:32:46Z

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

paulyoung · 2017-08-30T05:24:32Z

Is there anything else I can do here?

paf31 · 2017-08-31T23:19:14Z

src/Type/Row.purs

@@ -70,3 +74,17 @@ instance listToRowCons
  :: ( ListToRow tail tailRow
     , RowCons label ty tailRow row )
  => ListToRow (Cons label ty tail) row
+
+
+class Homogeneous (row :: # Type) fieldType


Also missing a functional dependency here.

Good catch.

Am I correct in thinking that the right thing to do here is | row -> fieldType?

paf31 · 2017-08-31T23:19:20Z

src/Type/Row.purs

@@ -70,3 +74,17 @@ instance listToRowCons
  :: ( ListToRow tail tailRow
     , RowCons label ty tailRow row )
  => ListToRow (Cons label ty tail) row
+
+
+class Homogeneous (row :: # Type) fieldType


Could you please add a comment?

paf31 · 2017-08-31T23:20:03Z

src/Type/Row.purs

@@ -70,3 +74,17 @@ instance listToRowCons
  :: ( ListToRow tail tailRow
     , RowCons label ty tailRow row )
  => ListToRow (Cons label ty tail) row
+
+
+class Homogeneous (row :: # Type) fieldType


Perhaps a value-level function fieldType :: RProxy row -> Proxy fieldType would be handy as well.

paf31 · 2017-08-31T23:21:51Z

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

LiamGoodacre · 2017-09-12T06:26:33Z

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 · 2017-09-12T06:47:07Z

Actually, I am mistaken. Ignore my previous concern.

LiamGoodacre · 2017-09-12T06:55:33Z

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

paulyoung · 2017-09-20T07:43:59Z

src/Type/Row.purs

@@ -70,3 +74,17 @@ instance listToRowCons
  :: ( ListToRow tail tailRow
     , RowCons label ty tailRow row )
  => ListToRow (Cons label ty tail) row
+
+-- | Ensure that every field in a row has the same type.
+class Homogeneous (row :: # Type) fieldType


I though I'd asked this already but can't see it now.

Is | row -> fieldType missing here?

Yes I think so

paulyoung · 2017-10-05T01:58:21Z

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

paf31 · 2017-11-19T20:05:35Z

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

Sorry for the delay. Yes please.

paf31 · 2017-11-20T17:33:38Z

src/Type/Row/Homogeneous.purs

+  , class Homogeneous
+  ) where
+
+-- | Ensure that every field in a row has the same type.


This comment should be removed, I think.

Yes. Strange things like this sometimes happen as a result of imports being managed by the PureScript Emacs layer.

paf31 · 2017-11-20T17:34:35Z

src/Type/Row/Homogeneous.purs

+     , FieldOf fields fieldType )
+  => Homogeneous row fieldType
+
+class FieldOf (rowList :: RowList) fieldType | rowList -> fieldType


It's not clear to me (without reading the docs quite a bit) which one of these should be used in regular code. I'd suggest naming this one something like HomogeneousRowList to just be explicit about it.

paf31 · 2017-11-20T17:35:34Z

src/Type/Row/Homogeneous.purs

+class FieldOf (rowList :: RowList) fieldType | rowList -> fieldType
+instance fieldOfCons
+  :: ( FieldOf tail fieldType
+     , TypeEquals fieldType fieldType2 )


Does this need TypeEquals? Can you just use

instance fieldOfCons :: FieldOf tail fieldType => FieldOf (Cons symbol fieldType tail) fieldType

I did that in 8b39b00#diff-86491d4ab8759f2c9d61332f39afdd9bR15 but you suggested using TypeEquals in #20 (comment) 😄

D'oh. Thanks :)

That was incorrectly copied by the PureScript Emacs layer.

To better communicate which class should be used in regular code.

paulyoung · 2017-12-06T23:20:10Z

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

paulyoung · 2017-12-06T23:20:59Z

src/Type/Row/Homogeneous.purs

+  :: ( HomogenousRowList tail fieldType
+     , TypeEquals fieldType fieldType2 )
+  => HomogenousRowList (Cons symbol fieldType tail) fieldType2
+instance fieldOfNil :: HomogenousRowList Nil fieldType


I suppose I should rename these to homogenousRowListCons and homogenousRowListNil

paf31 · 2017-12-10T23:48:00Z

Thanks!

Add RecordOf and FieldOf for homogeneous records

8b39b00

paulyoung commented Aug 10, 2017

View reviewed changes

Rename to Homogenous and move to Row type

17d4228

* Add `TypeEquals` constraint on field type * Add funcitonal dependency for `FieldOf`

paulyoung changed the title ~~Add RecordOf and FieldOf for homogeneous records~~ Add Homogeneous and FieldOf for homogeneous rows Aug 30, 2017

paf31 reviewed Aug 31, 2017

View reviewed changes

Add comment explaining Homogeneous class

0d25ad4

paulyoung commented Sep 20, 2017

View reviewed changes

Add missin fun dep

3d51ac4

paulyoung mentioned this pull request Oct 21, 2017

Creating maps from (homogeneous) records purescript-deprecated/purescript-maps#125

Open

Add Type.Row.Homogeneous

6d179f8

paf31 reviewed Nov 20, 2017

View reviewed changes

Remove unwanted comment

20fdbe6

That was incorrectly copied by the PureScript Emacs layer.

Rename FieldOf to HomogenousRowList

fe087b6

To better communicate which class should be used in regular code.

paulyoung commented Dec 6, 2017

View reviewed changes

Rename instances to coincide previous renaming

632538d

paulyoung changed the title ~~Add Homogeneous and FieldOf for homogeneous rows~~ Add Homogeneous and HomogeneousRowList for homogeneous rows Dec 6, 2017

paf31 merged commit 979f06a into purescript:master Dec 10, 2017

paulyoung deleted the paulyoung/homogeneous-records branch December 11, 2017 06:35

paulyoung mentioned this pull request Dec 11, 2017

Add map, traverse etc. purescript/purescript-record#19

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Add Homogeneous and HomogeneousRowList for homogeneous rows #20

Add Homogeneous and HomogeneousRowList for homogeneous rows #20

paulyoung commented Aug 10, 2017

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Add Homogeneous and HomogeneousRowList for homogeneous rows #20

Add Homogeneous and HomogeneousRowList for homogeneous rows #20

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