csharplang/meetings/2020/LDM-2020-02-10.md

C# Language Design for Feb. 10, 2020

Agenda

Records

Discussion

We're continuing our attempt to draw out the dependencies and individual features inside records.

When going through the list, what stands out is:

• Looking at with, we need to figure out what's mentionable in the with expression.

• We need to figure out exactly what we want for how primary constructors fit into records

There are a number of positives and negatives of primary constructors. On the negative side, a non-record primary constructor seems to consume syntactic space that could be used for records. If we think that records are the overwhelmingly common scenario, then it seems like using the shortest syntax for the most common feature is useful. On the positive side, primary constructors alone seem to support a simpler way of writing private implementation details. Separate from the value as a whole, there's some desire to have a special keyword just for records. That is, even if we didn't do primary constructors, it could be valuable to have an explicit modifier, like data to signify that this type has special behavior.

One possible pivot is to eliminate some composition syntax entirely, by creating a new type of declaration, record, e.g.

record Point(int X, int Y);

This would be equivalent to the syntax that we've been discussing with data, namely

data class Point(int X, int Y);

but since the class keyword is implied by default, the most common scenario would be just about as short as the shorter class Point(int X, int Y) form.

Conclusion

After taking everything into account, we think having an new keyword for records is good both for leaving space for non-record primary constructors, and also to serve as a clear signifier of record semantics.