Newbie here. Please excuse incorrect terminology I might use here ...
Below I can assign a field of a record at the top level at In, but I cannot do what looks like the very same assignment via a function call at In. Why is this? Yes, I see the "is not a symbol" error but don't understand it. Thanks!!
In:= r = rec["a",1]
In:= r[] = "b"
In:= f[x_] := x[] = "c"
During evaluation of In:= Set::setps: rec[b,1] in the part assignment is not a symbol. >>