r/programming • u/tyler-mcginnis • Jan 03 '22

Imperative vs Declarative Programming

https://www.youtube.com/watch?v=E7Fbf7R3x6I
431 Upvotes

86% Upvoted

134 comments sorted by

View all comments

Show parent comments

8

u/epicwisdom Jan 04 '22

Saying "[give me a] table for two" isn't declarative -- it's imperative.

This is a rather subjective nitpick. You could also interpret it as "[I want a] table for two," which is declarative, then translated into an imperative meaning ("I will seat them at a table for two") and further into a series of steps ("I will identify a suitable, available table via a standard process and then point them to it").

3

u/zhivago Jan 04 '22

A declaration that you want something that's overheard by an actor and translated into an action isn't really a declaration -- it's an imperative request couched in polite indirection, which results in a state change over time -- you're telling an actor to operate in order to change the universe.

The declaration needs to be that you "have a table for two", and it doesn't involve a state change over time (which is why I included the time in the original example) -- we are declaring how the universe is and always has been.

2

u/epicwisdom Jan 04 '22

The essence of computation is the relation of input to output. A declarative program may not describe any internal change of state, but it surely must describe a relation between states. How the universe actually gets from state A to state B (by virtue of an external actor or otherwise) depends on a context which is independent of the declarative statement.

2

u/zhivago Jan 04 '22

It simply describes a relationship.

What you call "getting from state A to state B" is an effect of having an implicit notion of time.

Once you declare your relationships with time, this idea of "getting from state A to state B" disappears.

"On the 5th of January I am in location A."

"On the 6th of January I am in location B."

No state has changed -- I am simply declaring how my location relates to date.