[Hugs-users] Some idioms for Trex

[Anthony Clayden]

Making best use of Trex needs a different mindset vs H98 datatypes with
named fields. (Or indeed vs named-fields data structures in many languages)
There's some examples around in the Trex/Hugs material, but I've seen
nothing showing this:

>    type WPoint r  = (x :: Float, y :: Float | r)            -- r extended
"with Point"
>    type WRadius r = (radius :: Float | r)
>    type WColour r = (colour :: String | r)
>    type ColouredCircle = Rec( WColour (WRadius (WPoint EmptyRow)))

The idea is rather than nest 'Point' as a datatype inside 'Circle' 'plug
and play' components (such as Point, Radius) to give a structural type.
It's interesting and impressive that the Wxxx synonyms take an argument of
kind row, return a result of kind row. `EmptyRow` in the last line is a
Hugs-defined Row constant that 'plugs' the end of the composed components,
just as `emptyRec` does for Trex Records.

I'd love to write that last type as

>    type MkRec r = Rec( r EmptyRow)
>    type ColouredCircle = MkRec (WColour . WRadius . WPoint)

But of course type synonyms -- even with parameters -- are not functions;
can't be composed; can't be passed to higher-order type synonyms.

Components like WPoint can be arguments to type-parametric functions for
record subtyping: such as a generic routine to 'move' any shape by
modifying its x, y coordinates. 'Move' in scare quotes because that's
returning a copy of the shape's data structure with modified coordinates,
not poking values into an object/reference.
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