Python is going to be hard

Chris Angelico rosuav at
Thu Sep 4 05:49:33 CEST 2014

On Thu, Sep 4, 2014 at 1:22 PM, Rustom Mody <rustompmody at> wrote:
> | Effect-free programming
> | -- Function calls have no side effects, facilitating compositional reasoning
> | -- Variables are immutable, preventing unexpected changes to program data by other code
> | -- Data can be freely aliased or copied without introducing unintended effects from mutation
> So to answer your question: print statements are side-effecting and therefore obstruct
> compositional reasoning.

Yeah, fine. One small problem: Computers aren't built to do nothing.
"Effect-free programming" is utterly valueless and purposeless. To
make a program actually useful, you need to have some effect
somewhere. So where do you do that? Where do you permit a function
with side effects?

If you force *everything* to be in the return value, you lose any
possibility of partial results - for instance, if it takes two weeks
to render a ray-traced animation, you actually spend the entire two
weeks building up a single, gigantic return value, which some outside
system (which is presumably allowed to have side effects) then saves
somewhere. In the meantime, you have absolutely no idea how far it's
progressed - no information that it's completed frame 6 and is working
on frame 7, nothing. Because telling the user anything is a side

And if my function f calls show_status_to_user() which has side
effects, then f has side effects too, and you can no longer reason
purely about f. The impurity that makes for practicality (hey, isn't
there something in the Zen of Python about that?) pollutes upwards
until all you'll have will be pockets of pure functional code that
execute quickly enough to not need any intermediate output. That
doesn't equate to "abolish print", that just means that you separate
the guts from the UI - which is good advice for any system, no matter
what its structure.


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