Std.HashMap.insertWith

fn

insertWith: &Std.HashMap.HashMap[K, V] -> K -> V -> fn(V, V) -> V -> Std.HashMap.HashMap[K, V] where K: Eq + Hash

This is a callable function.

A map with a pairing added, combining with any value already there.

The value already present is given first, so a caller writing an addition

or an append does not have to remember which way round the arguments came.

The text after the name is the type checked by Pudu.
Read arrows from left to right: inputs come first, and the final type is returned.
& borrows a value for this call instead of moving or copying it.
Names inside [ ] are type arguments, such as the item type held by a collection.