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Proc#compose ArgumentError incorrect #249
Description
Proc#compose raises an error when it shouldn't. The following two lambdas can compose:
make_twin = -> n { return n, n }
take_twin = -> n, m { puts n; puts m }
twin_comp = -> n { take_twin[*make_twin[n]] }
twin_comp[2]
2
2
=> nilBut with Proc#compose, it raises an ArgumentError:
take_twin.compose(make_twin)
ArgumentError: arity count mismatchOne solution is to wrap the inner call in a begin:
class Proc
def compose(g)
lambda do |*a|
begin
self[ *g[*a] ]
rescue ArgumentError => e
puts "#{e.class}: #{e.message}" # or whatever message is appropriate
puts e.backtrace
end
end
end
endThere's not really a way to check a proc's return arity at runtime, since it can return in more than one way.
Activity
trans commentedon Jan 9, 2016
Hmm... good point. Perhaps we can just get rid to the argument error condition altogether?
trans commentedon Nov 2, 2026
Proc#composeis deprecated (removal after 2027-09-30) and now just calls Ruby'sProc#<<, so Facets' arity check is gone. Ruby passes the inner result to the outer proc as one argument; for a multi-value return, splat explicitly:->(n){ take_twin[*make_twin[n]] }.