Question

I can’t wrap my head around the differences between sequence and LazyList. They’re both lazy and potentially infinite. While seq<'T> is IEnumerable<'T> from .NET framework, LazyList is included in F# PowerPack. In practice, I encounter sequences much more often than LazyLists.

What are their differences in terms of performance, usage, readability, etc? What are reasons for such a bad reputation of LazyList compared to that of seq?

Answer 1

LazyList computes each element only once regardless of how many times the list is traversed. In this way, it’s closer to a sequence returned from Seq.cache (rather than a typical sequence). But, other than caching, LazyList behaves exactly like a list: it uses a list structure under the hood and supports pattern matching. So you might say: use LazyList instead of seq when you need list semantics and caching (in addition to laziness).

Regarding both being infinite, seq‘s memory usage is constant while LazyList‘s is linear.

These docs may be worth a read.