[m-users.] Concise syntax for initialising and updating discriminated union types

Volker Wysk post at volker-wysk.de
Tue Jul 19 02:50:03 AEST 2022


There are two things which occur to me every now and then, about working
with discriminated union types with field names.

1. How to initialise a value of a discriminated union type, by using field
names (similar to the way they can be used for field access and field update

I find myself defining a "null" value, which is initialised without usage of
field names. But this is error prone. Afters, I change fields of that null
value to get my value.

This leads to point no. 2:

2. How to concisely update multiple fields? You can do this in multiple
steps, like this:

konstr1(LP3) :-
    LP1 = 'lp_link_target :='(null_link_parts, "Target"),
    LP2 = 'lp_link_subsection :='(LP1, yes("Subs.")),
    LP3 = 'lp_link_text :='(LP2, "bla").

But this too, is rather verbose and maybe error prone.

The code smells very much like state variables. It could be done like this:

konstr3(Out) :-
    some [!LP] (
        !:LP = null_link_parts,
        lp_link_target("Target", !LP),
        lp_link_subsection(yes("Subs."), !LP),
        lp_link_text("bla", !LP),
        Out = !.LP

For this, there need to be defined field update predicates:

lp_link_target(Val, In, Out)     :- Out = 'lp_link_target :='(In, Val).
lp_link_subsection(Val, In, Out) :- Out = 'lp_link_subsection :='(In, Val).
lp_link_text(Val, In, Out)       :- Out = 'lp_link_text :='(In, Val).

So there are two things that I'm missing. 

1. A concise syntax for initialising values of a discriminated union type.
Something like this (link_parts is a data constructor):

link_parts(lp_link_target = "Target",
           lp_link_subsection = yes("Subs."),    
           lp_link_text = "bla").

The compiler would complain when some fields aren't initialised.

2. Some concise way to update multiple fields, like above. Maybe the
compiler could additionally generate field update predicates for the fields
(like above), similar to field update functions.

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