4 ms·

i've never seen a programming language implement math-y notation in the vein of xs[0] + ... + xs[n] so that's kind of cool... though C++ seems like an odd p

by uryga 6y ago

i've never seen a programming language implement math-y notation in the vein of xs[0] + ... + xs[n] so that's kind of cool... though C++ seems like an odd place to find it. i wonder how expressive it is, i.e. what the limitations are. NOTE: the following might be incorrect, i really don't know C++ well enough to tell... unfortunately it looks like it's limited to working with "parameter packs", which i understand are basically varargs, and can't be used to fold/map e.g. a normal vector. which seems like a strange choice... if you're going to add this much syntax, why limit it like that?

somebodynew 6y ago
Fold expressions are expanded entirely at compile time to create a fixed-size expression that gets compiled normally (the number of terms in any given case is known before machine code is generated). Applying a fold over a std::vector would require runtime iteration (see std::accumulate). This is essentially a QoL improvement for existing users of variadic templates, where unnecessary complexity was required to simulate the behavior fold expressions provide for packs.
- uryga 6y ago
  i get that, i just think it'd be nice if they could be used for run-time stuff too – why create a gap between the compile- and run-time parts of the language? it seems like desugaring it into a loop should be pretty straightforward¹; it'd just be a concise replacement for some uses of std::accumulate. though as mentioned, i don't really use C++ much. are there reasons why this wouldn't work well for run-time stuff? --- ¹ it'd have to be mindful of short-circuiting constructs, but compilers special-case those anyway
  - jfkebwjsbx 6y ago
    It guess it would require a different syntax, otherwise it may not be clear how it was desugared (runtime vs. compile). Then again, we already have for loops, range for loops, the ranges library, the std algorithms, the parallel algorithms... I don't think people would welcome yet another way of writing things, but who knows...
    - uryga 6y ago
      > otherwise it may not be clear how it was desugared (runtime vs. compile) but does that really matter, as long it has well-defined semantics? even for the compile-time case, it's up to the compiler/optimizer to decide whether `(xs + ...)` ends up as multiple ADDs or one ADD in a loop – you're only guaranteed that it's observable behavior will be `xs[0] + xs[1] + xs[2] + ...`, no?
      - jfkebwjsbx 6y ago
        Yes, it does matter. When you are programming in a language like C++, you want to understand what the compiler generates for a given abstraction.
agumonkey 6y ago
python has ellispes in the grammar set but I don't think it's used yet. Also I'd bet a dollar that there are 100 lisp libs with this (published or not)
- uryga 6y ago
  seems like it's actually a common thing in Scheme (linked in a sibling comment)
  - agumonkey 6y ago
    yes
xfer 6y ago
Ellipsis being used for meta-programming expansion is decades old(used in scheme). Here is a reference: https://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29 https://docs.racket-lang.org/reference/stx-patterns.html#%28...
- uryga 6y ago
  thanks for pointing that out, completely forgot about lisps!

by uryga 6y ago

somebodynew 6y ago

Fold expressions are expanded entirely at compile time to create a fixed-size expression that gets compiled normally (the number of terms in any given case is known before machine code is generated). Applying a fold over a std::vector would require runtime iteration (see std::accumulate). This is essentially a QoL improvement for existing users of variadic templates, where unnecessary complexity was required to simulate the behavior fold expressions provide for packs.

uryga 6y ago
i get that, i just think it'd be nice if they could be used for run-time stuff too – why create a gap between the compile- and run-time parts of the language? it seems like desugaring it into a loop should be pretty straightforward¹; it'd just be a concise replacement for some uses of std::accumulate. though as mentioned, i don't really use C++ much. are there reasons why this wouldn't work well for run-time stuff? --- ¹ it'd have to be mindful of short-circuiting constructs, but compilers special-case those anyway
- jfkebwjsbx 6y ago
  It guess it would require a different syntax, otherwise it may not be clear how it was desugared (runtime vs. compile). Then again, we already have for loops, range for loops, the ranges library, the std algorithms, the parallel algorithms... I don't think people would welcome yet another way of writing things, but who knows...
  - uryga 6y ago
    > otherwise it may not be clear how it was desugared (runtime vs. compile) but does that really matter, as long it has well-defined semantics? even for the compile-time case, it's up to the compiler/optimizer to decide whether `(xs + ...)` ends up as multiple ADDs or one ADD in a loop – you're only guaranteed that it's observable behavior will be `xs[0] + xs[1] + xs[2] + ...`, no?
    - jfkebwjsbx 6y ago
      Yes, it does matter. When you are programming in a language like C++, you want to understand what the compiler generates for a given abstraction.

agumonkey 6y ago

python has ellispes in the grammar set but I don't think it's used yet. Also I'd bet a dollar that there are 100 lisp libs with this (published or not)

uryga 6y ago
seems like it's actually a common thing in Scheme (linked in a sibling comment)
- agumonkey 6y ago
  yes

xfer 6y ago

Ellipsis being used for meta-programming expansion is decades old(used in scheme). Here is a reference: https://docs.racket-lang.org/reference/stx-patterns.html#%28form._%28%28lib._racket%2Fprivate%2Fstxcase-scheme..rkt%29._......%29%29 https://docs.racket-lang.org/reference/stx-patterns.html#%28...

uryga 6y ago
thanks for pointing that out, completely forgot about lisps!