[ruby-core:90842] [Ruby trunk Feature#15477] Proc#arity returns -1 for composed lambda Procs of known arguments

Issue #15477 has been updated by mame (Yusuke Endoh).

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
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;
 
```

----------------------------------------
Feature #15477: Proc#arity returns -1 for composed lambda Procs of known arguments
https://bugs.ruby-lang.org/issues/15477#change-76031

* Author: robb (Robb Shecter)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
----------------------------------------
```
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.



-- 
https://bugs.ruby-lang.org/

Unsubscribe: <mailto:[e-mail]?subject=unsubscribe>
<http://lists.ruby-lang.org/cgi-bin/mailman/options/ruby-core>