[alkonaut]

The correct thing is imho to pass an immutable SomeType or an interface that only exposes the parts of SomeType necessary for the calculation and doesn’t allow mutation of the object.

Of course you don’t send around references to mutable objects and of course you only send to a function just what it needs - but that’s regardless of type system.

[jondubois]

Sometimes this will be the best approach possible but adhering with this principle too strongly can overcomplicate the general design/architecture - It can give developers a green light to start passing around complex types all over the place and harms the separation of concerns principle.

In terms of modularity and testability, the ideal architecture is when components communicate with each other in the simplest language (interface) possible. Otherwise you become too reliant on mocks during testing (which add brittleness and require more frequent test updates). I think very often, static typing can cause developers to become distracted from what is truly important; architecture and design philosophy. I think this is the core idea that Alan Kay (one of the inventors of OOP) has been trying to get across.

'I'm sorry that I long ago coined the term "objects" for this topic because it gets many people to focus on the lesser idea. The big idea is "messaging"' - Alan Kay

It's very clear from Alan Kay's writings that when he was talking about 'messaging' he was talking about communication between components and he did not intend for objects to be used in the place of messages.

[loup-vaillant]

Abandoning a feature just because it enables a misuse is the wrong way to do it in my opinion. Yes, some inexperienced, stubborn, stupid, or hurried developers will pass around complex types when they really shouldn't. But no, this drawback does not nullify the massive advantages of (good) static typing.

Sure, interfaces should be kept small. Let's to just that, then! Recognise that we want our classes/functions/modules to be deep (small interface/implementation ratio), and frown upon shallow instances in code reviews.

No need to give up static typing.

[andrekandre]

i think messaging is orthogonal to strong or weak typing (hence small/strongtalk, objc, self etc dont let you automatically coerce objects to different types than you expected) and those systems all use messaging