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This page describes the motivation and design for a refactoring of diagrams, to give them a semantics based on computing fixed points of functions from "context" information to raw tree of primitives together with some summary information.


1 Reference

See the original "manifesto" and ensuing mailing list discussion here:

See also later IRC discussion beginning here:

2 Detailed design

Most of the changes should be in the diagrams-core package, though a few things in diagrams-lib may need to change as well.

2.1 diagrams-core

No changes should be necessary to the following modules (all prefixed by Diagrams.Core): Envelope, HasOrigin, Juxtapose, Names, Points, Query, Trace, Transform, V.

I'm not sure about Style yet.

2.1.1 Diagrams.Core.Types

  • Change the name of UpAnnots to Summary.
  • Change the name of DownAnnots to Context,
  • and change its definition to
 type Context b v m = Style v ::: Name ::: SubMap b v m ::: ()

The addition of SubMap is so that we can use the positions of laid-out subdiagrams to compute the positions of others.

  • transfToAnnot and transfFromAnnot can be
  • deleted. QDiaLeaf and withQDiaLeaf can be
  • deleted.
  • Create new definitions
 type ContextualT b v q m a = ReaderT (Context b v q) m a
 type Contextual b v q a = ContextualT b v q Identity a

One could imagine making ContextualT a newtype but that would necessitate writing lots of instances for things like MonadState, MonadReader, and so on.

  • The definition of QDiagram should be changed to
 something like
 type QDiagram b v m = Contextual b v m (RTree b v Annotation,
 Summary b v m)

It's important that this is NOT a newtype, so we can freely use the Monad instance for Contextual when working with diagrams. This also means we don't need Wrapped or Rewrapped instances for it anymore.

  • The implementation of applyAnnotation needs to change.
 All it needs to do is create a new RTree node (an
 RAnnot node), ignoring the context and having no effect
 on the summary.  So something like
 applyAnnotation an = fmap (first (Node (RAnnot an) . (:[])))

We may end up having to do similar things in other situations so it may make sense to factor out parts of this into general utilities (e.g. addRoot :: RNode -> RTree -> RTree etc.)

  • Of course the implementation of almost all the rest of the functions
 in this module will need to change as well.  I'll walk through a few
 particular examples and then discuss more general principles.
 First, pointDiagram.  The current code is
 pointDiagram :: (Fractional (Scalar v), InnerSpace v) => Point v ->
              QDiagram b v m pointDiagram p = QD $ D.leafU (inj
              . toDeletable $ pointEnvelope p)

This creates a point envelope, wraps it in Deletable, injects it into an UpAnnots value, and uses it to create a leaf DUALTree. Everything up through UpAnnots still applies (though the name of UpAnnots has changed). What's different is that we aren't creating an explicit tree with summary values at leaves; we simply return the summary value as part of the result of the diagram function. So we can create a function leafS in parallel with leafU:

 emptyTree = Node REmpty []
 leafS :: Summary b v m -> QDiagram b v m leafS s = return
 (emptyTree, s)

Then the implementation of pointDiagram only needs to change to

 pointDiagram p = leafS (inj . toDeletable $ pointEnvelope p)
  • The current type of getU' can't work anymore, since we
 can't escape the Contextual monad.  But we can have
 getU' :: (Monoid u', u :>: u') => QDiagram b v m -> Contextual b v m
 u' getU' = fmap (option mempty id . get . snd)
  • setEnvelope: the main issue here is to figure out
 suitable replacements for applyUPre and
 applyUPost.  We could do
 applySPre :: Summary b v m -> QDiagram b v m -> QDiagram b v m
 applySPre s = (fmap . second) (s<>)

But note we can also give this a much more general type signature like

 applyUPre :: (Functor f, Semigroup s) => s -> f (t,s) -> f (t,s)

Not sure which is better.

In any case, we don't need any of the over _Wrapped stuff any more.

  • envelope is problematic. We had
 envelope :: forall b v m. (OrderedField (Scalar v), InnerSpace v ,
                           HasLinearMap v, Monoid' m) => Lens'
                           (QDiagram b v m) (Envelope v)

But this type won't work any more, because e.g. we can't simply extract an Envelope from a QDiagram. I suppose we could do

 ... Lens' (QDiagram b v m) (Contextual b v m (Envelope v))


  • trace and setTrace are similar.
  • Name stuff: think about monad, etc. subMap,
 names, nameSub, lookupName,
  • withName, withNameAll,
 withNames: should be able to simplify these to take
 advantage of monad.  Don't need to have them in CPS anymore.
  • query, sample, value

2.1.2 Diagrams.Core.Compile

2.2 diagrams-lib