"The essence of functional programming is treating a program as evaluating a function without carrying around state"
Isn't a closure a function that carries around state? There's an old tale about this:
"The venerable master Qc Na was walking with his student, Anton. Hoping to
prompt the master into a discussion, Anton said "Master, I have heard that
objects are a very good thing - is this true?" Qc Na looked pityingly at
his student and replied, "Foolish pupil - objects are merely a poor man's
closures."
Chastised, Anton took his leave from his master and returned to his cell,
intent on studying closures. He carefully read the entire "Lambda: The
Ultimate..." series of papers and its cousins, and implemented a small
Scheme interpreter with a closure-based object system. He learned much, and
looked forward to informing his master of his progress.
On his next walk with Qc Na, Anton attempted to impress his master by
saying "Master, I have diligently studied the matter, and now understand
that objects are truly a poor man's closures." Qc Na responded by hitting
Anton with his stick, saying "When will you learn? Closures are a poor man's
object." At that moment, Anton became enlightened."
> Isn't a closure a function that carries around state?
Well, that's the question, isn't it?
In our present context, to me "state" suggests something persistent and independent, something that could be read and possibly written by multiple paths.
Do you view creating a closure as customising an existing function with specific data, or as defining a whole new function? I think of it as the latter, and it just happens to be at run-time. The resulting function is still pure, and any customisation based on particular data is an integral part of it that isn't externally visible, which doesn't feel "stateful".
Isn't a closure a function that carries around state? There's an old tale about this:
"The venerable master Qc Na was walking with his student, Anton. Hoping to prompt the master into a discussion, Anton said "Master, I have heard that objects are a very good thing - is this true?" Qc Na looked pityingly at his student and replied, "Foolish pupil - objects are merely a poor man's closures."
Chastised, Anton took his leave from his master and returned to his cell, intent on studying closures. He carefully read the entire "Lambda: The Ultimate..." series of papers and its cousins, and implemented a small Scheme interpreter with a closure-based object system. He learned much, and looked forward to informing his master of his progress.
On his next walk with Qc Na, Anton attempted to impress his master by saying "Master, I have diligently studied the matter, and now understand that objects are truly a poor man's closures." Qc Na responded by hitting Anton with his stick, saying "When will you learn? Closures are a poor man's object." At that moment, Anton became enlightened."
http://people.csail.mit.edu/gregs/ll1-discuss-archive-html/m...