6 ms·

It's really surprising that it's rare in functional languages. Immutability seems like it should guarantee no cycles (?), so reference counting could be used.

jbjohns 5y ago
Why would immutability guarantee no cycles? Here is a line of valid haskell: star e = let (sp, a) = (Split a e, atom sp) in sp EDIT: I guess I should probably explain it: star is a function that takes an expression "e" and returns the value "sp" which is "Split a e" where "a" is the results of calling the "atom" function on "sp". This is creating a representation of a regex star operator. Note that the tuple defined in the let definition is only to define a name for the two values of the tuple so that they can refer to each other.
- JoelMcCracken 5y ago
  I mean generally tying the knot is a useful technique, but I think these scenarios all require/exploit non-strict, which is in itself not really immutable in the sense most people use it. But yes, such code is often useful so that e.g. a parent xml node can refer to its childen nodes while also children nodes can refer to their parents. Anyway, I'm not sure about this, but I think you can't have circular data structures in the context of strict evaluation (or can you? maybe by defering execution via anonymous functions? I wonder....)
  - jbjohns 5y ago
    I'm fairly certain I've done similar things in Ocaml (in fact, I think it's where I learned this technique).
wyager 5y ago
There are two ways to get cycles in Haskell. One is through “tying the knot”. E.g. to create an infinite list of 1,1,1,1,1,… ones = 1 : ones This will actually be compiled to a list cell with a pointer back to itself. You can construct more complicated self-referential data structures thanks to laziness. The other way you could get a cycle is that it actually does have mutable data structures, although their use is restricted so they can’t have any observable mutable effect in pure code. But you have e.g. IORef which is basically a mutable pointer. If you wanted no cycles you would need to eliminate some subset of laziness, knot-tying transformations, recursion, and any support for mutable data structures. But yes, I think it could be done.
kragen 5y ago
Reference counting is usually very expensive, because even reading a variable updates the reference count, and ending a scope involves testing the reference count of every variable defined inside the scope and conditionally deallocating the referent. Without reference counting, here's the end of a hairy function scope that deallocates 15 local variables and restores two callee-saved registers: 11a6: 48 83 c4 78 add $0x78,%rsp 11aa: 5b pop %rbx 11ab: 41 5e pop %r14 11ad: c3 retq Now imagine looping over 15 local variables to decrement each reference count, test it, and do a conditional jump based on the result; if that's open-coded, your epilogue is humongous, and if it's in a reference-count-decrementing subroutine, it's going to have mispredicted branches all over the place, costing you maybe 15 cycles each, a total of about 100 cycles for this function. We're talking about adding an order of magnitude of cost to subroutine call, or more. (I think this function doesn't really need 15 local variables; that's the compiler's fault.) This gets worse with multithreading, because writing to the same reference count on different cores would even in the best case require one core stealing the cache line from another in order to modify it, which may stall the core; but often even an atomic reference count increment is more expensive even than that because it involves a memory barrier. Reference counting can become reasonably cheap if it's done at large granularity (filesystem files or COM objects, not conses); if you can elide almost all of the reference-count updates, as in Rust; or if your language runtime is just so dog-slow at everything that the extra cost of reference counting isn't that important, like CPython. 30 years ago or more, before generational GC had gone mainstream, reference counting was a more reasonable choice, because GC was going to be very slow in any case, and ref counting at least used less memory—especially important on machines without cache or virtual memory. (Purely immutable (applicative) languages like Haskell and Miranda are usually lazy, since that's the payoff for completely abjuring side effects. But lazy evaluation is implemented at the machine level by mutation.)
- naasking 5y ago
  > Reference counting is usually very expensive, because even reading a variable updates the reference count There are many papers out there on how to elide most ref count operations on locals, and runtimes that use ref counting for automatic memory management typically defer ref count updates in various clever ways (like Nim IIRC). You give up some latency for significant throughput gains.
dwohnitmok 5y ago
The big thorn is closures, which show up all over the place in FP. You either need to limit closures vs ordinary functions (as e.g. Rust does), have manual memory annotations (such as e.g. what Swift does) or you basically need a GC. The former two choices are annoying if you really take advantage of functions as first-class citizens.
- silon42 5y ago
  Yes, I thought that might be a problem... I guess Rust is the choice for me.

jbjohns 5y ago

Why would immutability guarantee no cycles? Here is a line of valid haskell: star e = let (sp, a) = (Split a e, atom sp) in sp EDIT: I guess I should probably explain it: star is a function that takes an expression "e" and returns the value "sp" which is "Split a e" where "a" is the results of calling the "atom" function on "sp". This is creating a representation of a regex star operator. Note that the tuple defined in the let definition is only to define a name for the two values of the tuple so that they can refer to each other.

JoelMcCracken 5y ago
I mean generally tying the knot is a useful technique, but I think these scenarios all require/exploit non-strict, which is in itself not really immutable in the sense most people use it. But yes, such code is often useful so that e.g. a parent xml node can refer to its childen nodes while also children nodes can refer to their parents. Anyway, I'm not sure about this, but I think you can't have circular data structures in the context of strict evaluation (or can you? maybe by defering execution via anonymous functions? I wonder....)
- jbjohns 5y ago
  I'm fairly certain I've done similar things in Ocaml (in fact, I think it's where I learned this technique).

wyager 5y ago

There are two ways to get cycles in Haskell. One is through “tying the knot”. E.g. to create an infinite list of 1,1,1,1,1,… ones = 1 : ones This will actually be compiled to a list cell with a pointer back to itself. You can construct more complicated self-referential data structures thanks to laziness. The other way you could get a cycle is that it actually does have mutable data structures, although their use is restricted so they can’t have any observable mutable effect in pure code. But you have e.g. IORef which is basically a mutable pointer. If you wanted no cycles you would need to eliminate some subset of laziness, knot-tying transformations, recursion, and any support for mutable data structures. But yes, I think it could be done.

kragen 5y ago

Reference counting is usually very expensive, because even reading a variable updates the reference count, and ending a scope involves testing the reference count of every variable defined inside the scope and conditionally deallocating the referent. Without reference counting, here's the end of a hairy function scope that deallocates 15 local variables and restores two callee-saved registers: 11a6: 48 83 c4 78 add $0x78,%rsp 11aa: 5b pop %rbx 11ab: 41 5e pop %r14 11ad: c3 retq Now imagine looping over 15 local variables to decrement each reference count, test it, and do a conditional jump based on the result; if that's open-coded, your epilogue is humongous, and if it's in a reference-count-decrementing subroutine, it's going to have mispredicted branches all over the place, costing you maybe 15 cycles each, a total of about 100 cycles for this function. We're talking about adding an order of magnitude of cost to subroutine call, or more. (I think this function doesn't really need 15 local variables; that's the compiler's fault.) This gets worse with multithreading, because writing to the same reference count on different cores would even in the best case require one core stealing the cache line from another in order to modify it, which may stall the core; but often even an atomic reference count increment is more expensive even than that because it involves a memory barrier. Reference counting can become reasonably cheap if it's done at large granularity (filesystem files or COM objects, not conses); if you can elide almost all of the reference-count updates, as in Rust; or if your language runtime is just so dog-slow at everything that the extra cost of reference counting isn't that important, like CPython. 30 years ago or more, before generational GC had gone mainstream, reference counting was a more reasonable choice, because GC was going to be very slow in any case, and ref counting at least used less memory—especially important on machines without cache or virtual memory. (Purely immutable (applicative) languages like Haskell and Miranda are usually lazy, since that's the payoff for completely abjuring side effects. But lazy evaluation is implemented at the machine level by mutation.)

naasking 5y ago
> Reference counting is usually very expensive, because even reading a variable updates the reference count There are many papers out there on how to elide most ref count operations on locals, and runtimes that use ref counting for automatic memory management typically defer ref count updates in various clever ways (like Nim IIRC). You give up some latency for significant throughput gains.

dwohnitmok 5y ago

The big thorn is closures, which show up all over the place in FP. You either need to limit closures vs ordinary functions (as e.g. Rust does), have manual memory annotations (such as e.g. what Swift does) or you basically need a GC. The former two choices are annoying if you really take advantage of functions as first-class citizens.

silon42 5y ago
Yes, I thought that might be a problem... I guess Rust is the choice for me.