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Feature #15477
open
Proc#arity returns -1 for composed lambda Procs of known arguments
Feature #15477: Proc#arity returns -1 for composed lambda Procs of known arguments
Status:
Open
Assignee:
-
Target version:
-
Description
f = -> x { x + 2 } g = -> x { x * 2 } h = f << g f.arity # => 1 g.arity # => 1 h.arity # => -1 THIS SHOULD BE 1 because h "knows" that it takes exactly 1 argument: h.call # => ArgumentError (given 0, expected 1) Lambda Procs which are composed using << seem to partially lose knowledge of their arity. I don't know if this affects other procs, or the >> operator as well. The Proc#arity docs state that -1 is returned only when a variable or unknown number of arguments are expected by the Proc. But here, that's not the case.
Updated by mame (Yusuke Endoh) almost 7 years ago
- Tracker changed from Bug to Feature
- ruby -v deleted (
ruby 2.6.0p0 (2018-12-25 revision 66547) [x86_64-linux]) - Backport deleted (
2.4: UNKNOWN, 2.5: UNKNOWN, 2.6: UNKNOWN)
Looks not a bug to me. Moving to the feature tracker.
A patch is attached.
diff --git a/proc.c b/proc.c index c09e845ec0..45e2a21551 100644 --- a/proc.c +++ b/proc.c @@ -3063,6 +3063,16 @@ compose(VALUE dummy, VALUE args, int argc, VALUE *argv, VALUE passed_proc) return rb_funcallv(f, idCall, 1, &fargs); } +static VALUE +compose_proc_new(VALUE procs) +{ + VALUE first_proc = RARRAY_AREF(procs, 1); + int max_arity, min_arity = rb_proc_min_max_arity(first_proc, &max_arity); + int lambda_p = rb_proc_lambda_p(first_proc); + struct vm_ifunc *ifunc = rb_vm_ifunc_new((rb_block_call_func_t) compose, (void *)procs, min_arity, max_arity); + return cfunc_proc_new(rb_cProc, (VALUE)ifunc, lambda_p); +} + /* * call-seq: * prc << g -> a_proc @@ -3089,7 +3099,7 @@ proc_compose_to_left(VALUE self, VALUE g) GetProcPtr(self, procp); is_lambda = procp->is_lambda; - proc = rb_proc_new(compose, args); + proc = compose_proc_new(args); GetProcPtr(proc, procp); procp->is_lambda = is_lambda; @@ -3122,7 +3132,7 @@ proc_compose_to_right(VALUE self, VALUE g) GetProcPtr(self, procp); is_lambda = procp->is_lambda; - proc = rb_proc_new(compose, args); + proc = compose_proc_new(args); GetProcPtr(proc, procp); procp->is_lambda = is_lambda; 
Updated by matz (Yukihiro Matsumoto) almost 7 years ago
Yes, please.
Matz.
Updated by majjoha (Mathias Jean Johansen) over 6 years ago
For what it is worth, this appears to be an issue when dealing with curried procs as well.
curried_proc = ->(a, b) { a + b }.curry # => <Proc:0x00007fa7698e7700 (lambda)> first = curried_proc.(1) # => <Proc:0x00007fa76991a0d8 (lambda)> curried_proc.arity # => -1 first.arity # => -1 Actions