Advanced types interview questions

Mid to senior

You are modelling a filter clause with And, Or, Not and primitives such as Equals and Contains. Every primitive takes string arguments and every clause evaluates to a bool. A teammate wants a GADT; another argues for a plain variant. How would you decide?

What it tests

Picking the shape by what the compiler would refine with the index, rather than by habit; weighing the extra text and messages against the compile-time guarantees.

First step

List what the GADT index would carry here: with every primitive taking a string and every clause evaluating to bool, the index would be bool for every constructor. The GADT would then rule out no case the plain variant would allow, so the extra apparatus pays for nothing.

Follow-ups

• What would change if primitives also took ints or booleans, and the evaluator should return the column's type?
• How would you write a well-formedness check on each variant, and what does the plain variant make easier?
• When would you surface an error to the user versus letting the type system catch it?

From the lesson Generalised algebraic data types (GADTs)