There is a bit of confusion around the word "understand" here. It is sensible to think of an object as "understanding" a message or not, but in fact the selector that tells you whether an object understand a message is #respondsTo:, while the equivalent on the class which tells you whether instances of that class understand is the one that's actually called #canUnderstand:. Other Smalltalk dialects simply say "ReadWriteStream does not understand #selector", no "a[n]" or "Instance of". (I like the use of present tense "does not" instead of past-tense "did not", as well, but that's a much more nitpicky personal preference.) This is in fact already what you are doing with your class-side example, as `PragmaMenuBuilder class` is in fact not the class (that's just `PragmaMenuBuilder`), but the class *of* the class, itself an instance of `Metaclass`. On Fri, Jun 27, 2025 at 4:45 PM Aik-Siong Koh <askoh@askoh.com> wrote:

"nil did not understand #selector" is great.

Similarly, "Instance of " can be replace by "a" as in "aObject did not understand #selector" "aReadWriteStream did not understand #selector"

For classes "PragmaMenuBuilder class did not understand #selector"

Daniel Slomovits wrote:

I think it's because nil *is* pretty special--this is the equivalent of a NullPointerException or similar in other languages (even though in fact nil is an object like any other, it *is* usually a different type of *semantic* error on the part of the programmer than with any other class). UndefinedObject is also a pretty long class name, so it's nice to leave that out. But I would be all in favor of making it "nil did not understand #selector", to be more consistent but still concise.

On Fri, Jun 27, 2025, 3:00 PM Aik-Siong Koh <askoh@askoh.com> wrote:

...

Why is UndefinedObject version of "did not understand" so different from others? It also inverts the object message paradigm. Thanks Aik-Siong Koh

description

"Returns a textual description of the exception (based on the message instance variable). If message is nil, it returns the defaultDescription instead."

message ifNil: [ ^ self defaultDescription ]. message lookupClass == UndefinedObject ifTrue: [ ^ message selector printString , ' was sent to nil' ].

^ 'Instance of ' , message lookupClass printString , ' did not understand ' , message selector printString