Re: Undefined in LISP

Daniel Cerqueira <dan.list@lispclub.com> writes:

 Should an undefined expression be represented by a symbol, or should
an undefined expression be silent (without returning a symbol)?

Symbols are values in Lisp, so if an "undefined expression" returns a
symbol, then it's not undefined.  That symbol is the definition.

Can you give an example of an undefined expression?  In denotational
semantics, a divergent expression is represented by the symbol ⊥ ("bottom",
_|_ if the unicode glyph didn't show up for you).  That's what happens
if you try to evaluate something like 1/0.  Bottom is not a value that
is returned, but rather, it means the program fails.  The runtime could
handle it as a crash, an exception being raised, an infinite loop, etc.
Those are all examples of divergence.

I don't know if that is helpful for what you were asking.  See also:

https://en.wikibooks.org/wiki/Haskell/Denotational_semantics