4 ms·

That works for tail recursive functions, but not tail calls. TCO works for all tail calls, not just the recursive ones, so that jump would be to another functi

That works for tail recursive functions, but not tail calls. TCO works for all tail calls, not just the recursive ones, so that jump would be to another function's body. The current stack frame may not have enough space for the new function's.

poizan42 10y ago
It's true, but you probably don't want TCO for all tail calls since you lose information in stack traces. As long as you have control over all the code generated you can in principle compile multiple functions into a single one and just add some stubs for interop purposes. This of course complicates how you track function pointers (or equivalently) and may have some overhead. Still the cases where you can use recur today could be done automatically by the compiler without much trouble.
- jeremiep 10y ago
  Good point! One worry I have about automatic TCO where recur/loop currently is used is with Lisp's dynamic nature. What happens to everyone who has a reference to the fn when you redefine its def? You're basically switching (defn f [] (recur [] (loop))) to (defn f [] (f)). What if I do (def g f) and then (def f +)? Will the original f now recur into + when I call (g)? If it still calls into the original f then that creates a mismatch between reloading normal and recur'd functions adding complexity to a language striving for simplicity. I'm probably overthinking it :)
  - poizan42 10y ago
    If you want late binding at the call site it's probably unavoidable to have some extra complexity anyways, what if I changed f into something that wasn't callable? However Clojure binds at the definition time, e.g. > (defn f [] 42) #'sandbox9516/f > (f) 42 > (def g f) #'sandbox9516/g > (g) 42 > (def f 1337) #'sandbox9516/f > (g) 42 > (f) java.lang.ClassCastException: java.lang.Long cannot be cast to clojure.lang.IFn
    - jeremiep 10y ago
      That's because defn binds f to the _value_ of the function, while (f) binds to the _variable_. > (defn f [] (f)) #'sandbox9672/f > (def g f) #'sandbox9672/g > (def f 42) #'sandbox9672/f > (g) java.lang.ClassCastException: java.lang.Long cannot be cast to clojure.lang.IFn
    - deleted 10y ago
      [deleted]

poizan42 10y ago

It's true, but you probably don't want TCO for all tail calls since you lose information in stack traces. As long as you have control over all the code generated you can in principle compile multiple functions into a single one and just add some stubs for interop purposes. This of course complicates how you track function pointers (or equivalently) and may have some overhead. Still the cases where you can use recur today could be done automatically by the compiler without much trouble.

jeremiep 10y ago
Good point! One worry I have about automatic TCO where recur/loop currently is used is with Lisp's dynamic nature. What happens to everyone who has a reference to the fn when you redefine its def? You're basically switching (defn f [] (recur [] (loop))) to (defn f [] (f)). What if I do (def g f) and then (def f +)? Will the original f now recur into + when I call (g)? If it still calls into the original f then that creates a mismatch between reloading normal and recur'd functions adding complexity to a language striving for simplicity. I'm probably overthinking it :)
- poizan42 10y ago
  If you want late binding at the call site it's probably unavoidable to have some extra complexity anyways, what if I changed f into something that wasn't callable? However Clojure binds at the definition time, e.g. > (defn f [] 42) #'sandbox9516/f > (f) 42 > (def g f) #'sandbox9516/g > (g) 42 > (def f 1337) #'sandbox9516/f > (g) 42 > (f) java.lang.ClassCastException: java.lang.Long cannot be cast to clojure.lang.IFn
  - jeremiep 10y ago
    That's because defn binds f to the _value_ of the function, while (f) binds to the _variable_. > (defn f [] (f)) #'sandbox9672/f > (def g f) #'sandbox9672/g > (def f 42) #'sandbox9672/f > (g) java.lang.ClassCastException: java.lang.Long cannot be cast to clojure.lang.IFn
  - deleted 10y ago
    [deleted]