Some idioms for Trex

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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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<div dir=3D"ltr">Making best use of Trex needs a different mindset vs H98 d=
atatypes=C2=A0with named fields. (Or indeed vs named-fields data structures=
 in many languages) There&#39;s some examples around in the Trex/Hugs mater=
ial, but I&#39;ve seen nothing showing this:<div><br></div><div>&gt;=C2=A0 =
=C2=A0 type WPoint r =C2=A0=3D (x :: Float, y :: Float | r) =C2=A0 =C2=A0 =
=C2=A0 =C2=A0 =C2=A0 =C2=A0-- r extended &quot;with Point&quot;<br>&gt;=C2=
=A0 =C2=A0 type WRadius r =3D (radius :: Float | r)<br>&gt;=C2=A0 =C2=A0 ty=
pe WColour r =3D (colour :: String | r)<br>&gt;=C2=A0 =C2=A0 type ColouredC=
ircle =3D Rec( WColour (WRadius (WPoint EmptyRow)))<br></div><div><br></div=
><div>The idea is rather than nest &#39;Point&#39; as a datatype inside &#3=
9;Circle&#39; &#39;plug and play&#39; components (such as Point, Radius) to=
 give a structural type. It&#39;s interesting and impressive that the Wxxx=
=C2=A0synonyms take an argument of kind row, return a result of kind row. `=
EmptyRow` in the last line is a Hugs-defined Row constant that &#39;plugs&#=
39; the end of the composed components, just as `emptyRec` does for Trex Re=
cords.</div><div><br></div><div>I&#39;d love to write that last type as</di=
v><div><br></div><div>&gt;=C2=A0 =C2=A0 type MkRec r =3D Rec( r EmptyRow)</=
div><div>&gt;=C2=A0 =C2=A0 type ColouredCircle =3D MkRec (WColour . WRadius=
 . WPoint)</div><div><br></div><div>But of course type synonyms -- even wit=
h parameters -- are not functions; can&#39;t be composed; can&#39;t be pass=
ed to higher-order type synonyms.</div><div><br></div><div>Components like =
WPoint can be arguments to type-parametric functions for record subtyping: =
such as a generic routine to &#39;move&#39; any shape by modifying its x, y=
 coordinates. &#39;Move&#39; in scare quotes because that&#39;s returning a=
 copy of the shape&#39;s data structure with modified coordinates, not poki=
ng values into an object/reference.</div></div>

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