On 1/14/2019 9:56 AM, Laurent wrote:
I'm not entirely sure where I got that from, but to me a procedure is
a ("impure") function with possible side effects (printing, mutating a
global variable, sending emails, etc.). By contrast, in any given
context, a function is (again, to me) synonymous with "pure
function"(*) and can be called with the same values and will always
return the same value, and could in principle be substituted with any
other function implementating the same input-output specification
without changing the meaning of the whole program.
(*) https://en.wikipedia.org/wiki/Pure_function
There is at least one language where the "subroutines" can't return
values ... so what would you call them?
[Before you ask, the answer is Prolog.]
I've been programming [in a variety of languages] since the early 80s,
and I've never heard this notion that "procedure" was synonymous with
"impure function". If you look through old texts, what distinguished
procedures from functions was the *absence* of return values - which in
turn meant that a procedure could do useful work only through side effects.
historically, most languages have conflated the two notions -
syntactically offering a single impure form that can be either a
function or a procedure. But for the compiler - even with impurity -
there can be considerable benefit to distinguish the form that returns
values from the form that does not. [Again, I would encourage people
to study compiler texts - there's a lot of benefit to seeing how things
are really done.]
Potentially, at least, there is even more benefit to knowing a function
is pure - in theory at least a call with a given set of arguments could
simply be replaced by the result. But in practice, pure functions
rarely are evaluated at compile time - far more often results computed
at runtime are cached in case the same arguments are seen again.
George
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