search

getify / Functional-Light-JS

Pragmatic, balanced FP in JavaScript. @FLJSBook on twitter.
http://FLJSBook.com
Other
16.64k stars 1.96k forks source link

What about a program the only does computation? #77

Closed adriano-di-giovanni closed 7 years ago

adriano-di-giovanni commented 7 years ago

In chapter 5 you say:

The punchline to this chapter: it's impossible to write a program with no side effects. Well, not impossible; you certainly can. But that program won't do anything useful or observable. If you wrote a program with zero side effects, you wouldn't be able to tell the difference between it and a deleted or empty program.

I think that a good example of a computer program with no side-effects is the one of a program that only does computation.

getify commented 7 years ago

What kind of computation? And how would you see or observe the outcome of said program?Would it be in a file or on the screen? All the ways you observe program output are side effects.

adriano-di-giovanni commented 7 years ago

I think that a REST API server with a pure endpoint is a good example. Isn't it?

getify commented 7 years ago

No. Network communication is most definitely a side effect. All I/O of any form is a side effect, this is a pretty well-established notion.

That doesn't mean I/O is bad, it just means that since I/O is necessary (and is a side-effect), then programs can never be fully side-effect free.

adriano-di-giovanni commented 7 years ago

Is network I/O a side effect even if it's only interface to a pure function?

getify commented 7 years ago

Side effects are context sensitive. Inside the API, the function is pure, but when you consider the entire system (basically, two distinct entities -- client and server -- that are communicating over the network), it's not. A packet being transmitted over a network interface is an indirect change in the overall state of this system.

adriano-di-giovanni commented 7 years ago

I undestand but you say that

the purity of a function is judged from the outside, regardless of what goes on inside.

There's something I can't grasp. A subtle difference that's not evident to me.

getify commented 7 years ago

Function Purity obeys this law: referential transparency. If the function call can be replaced with its output, and the overall behavior of the system will work exactly the same, it's pure.

In the bigger context of a system including network communication, you couldn't replace a network call with its response without affecting the system, given that network packets have to get exchanged and that changes the state of the internet as a result.

adriano-di-giovanni commented 7 years ago

I understand. Thanks.