Question

Something that comes up a fair amount when dealing with heterogeneous data is the need to partially change simple objects that hold data. For instance, you might want to add, drop, or rename a property, or concatenate two objects. This is easy enough in dynamic languages, but I’m wondering if there are any clever solutions proposed by static languages?

To fix ideas, are there any languages that enable, perhaps through some sort of static mixin syntax, something like this (C#):

var hello = new { Hello = "Hello" };
var world = new { World = "World" };
var helloWorld = hello + world;
Console.WriteLine(helloWorld.ToString());
//outputs {Hello = Hello, World = World}

This certainly seems possible, since no runtime information is used. Are there static languages that have this capability?

Added:

A limited version of what I’m considering is F#’s copy-and-update record expression:

let myRecord3 = { myRecord2 with Y = 100; Z = 2 }

Answer 1

What you’re describing is known in programming language research as record concatenation. There has been some work on static type systems for record concatenation, mostly in the context of automatic type inference a la Haskell or ML. To the best of my knowledge it has not yet made an impact on any mainstream programming languages.