Question

When do you choose to type a given function’s return type as Seq vs Iterable vs Traversable (or alternatively even deeper within Seq‘s hierarchy)?

How do you make that decision? We have a lot of code that returns Seqs by default (usually starting from results of a DB query and successive transformations). I tend to want to make the return types Traversable by default and Seq when specifically expecting a given order. But I don’t have a strong justification for doing so.

I am perfectly familiar with the definition of each trait, so please don’t answer with defining the terms.

Answer 1

This is a good question. You have to balance two concerns:

• (1) try to keep your API general, so you can change the implementation later
• (2) give the caller some useful operations to perform on the collection

Where (1) asks you to be as little specific about the type (e.g. Iterable over Seq), and (2) asks you the opposite.

Even if the return type is just Iterable, you can still return let’s say a Vector, so if the caller wishes to gain extra power, it can just call .toSeq or .toIndexedSeq on it, and that operation is cheap for a Vector.

As a measure of the balance, I would add a third point:

• (3) use a type that kind of reflects how the data is organised. E.g. when you can assume that the data does have a sequence, give Seq. If you can assume that no two equal objects can occur, give a Set. Etc.

Here are my rules of thumb:

• try to use only a small set of collections: Set, Map, Seq, IndexedSeq
• I often violate this previous rule, though, using List in favour of Seq. It allows the caller to do pattern matching with the cons extractors
• use immutable types only (e.g. collection.immutable.Set, collection.immutable.IndexedSeq)
• do not use concrete implementations (Vector), but the general type (IndexedSeq) which gives the same API
• if you are encapsulating a mutable structure, only return Iterator instances, the caller can then easily generate a strict structure, e.g. by calling toList on it
• if your API is small and clearly tuned towards “big data throughput”, use IndexedSeq

Of course, this is my personal choice, but I hope it sounds sane.