Question

So I’ve got a fairly deep hierarchy of record definitions:

-record(cat,            {name = '_',           attitude = '_',}). -record(mat,            {color = '_',          fabric = '_'}). -record(packet,         {cat = '_',            mat = '_'}). -record(stamped_packet, {packet = '_',         timestamp = '_'}). -record(enchilada,  {stamped_packet = '_', snarky_comment = ''}). 

And now I’ve got an enchilada, and I want to make a new one that’s just like it except for the value of one of the subsubsubrecords. Here’s what I’ve been doing.

update_attitude(Ench0, NewState)   when is_record(Ench0, enchilada)->      %% Pick the old one apart.     #enchilada{stamped_packet     = SP0} = Ench0,     #stamped_packet{packet = PK0} = SP0,     #packet{cat = Tag0}    = PK0,      %% Build up the new one.     Tude1 = Tude0#cat{attitude = NewState},     PK1 = PK0#packet{cat = Tude1},     SP1 = SP0#stamped_packet{packet = PK1},      %% Thank God that's over.     Ench0#enchilada{stamped_packet = SP1}. 

Just thinking about this is painful. Is there a better way?

Answer 1

As Hynek suggests, you can elide the temporary variables and do:

update_attitude(E = #enchilada{stamped_packet = (P = #packet{cat=C})},                 NewAttitude) ->     E#enchilada{stamped_packet = P#packet{cat = C#cat{attitude=NewAttitude}}}. 

Yariv Sadan got frustrated with the same issue and wrote Recless, a type inferring parse transform for records which would allow you to write:

-compile({parse_transform, recless}).  update_attitude(Enchilada = #enchilada{}, Attitude) ->     Enchilada.stamped_packet.packet.cat.attitude = Attitude.