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<  Erlang  ~  programmatic method to build a record
5hundy
Posted: Tue Nov 11, 2008 10:41 pm Reply with quote
User Joined: 14 Oct 2007 Posts: 17
Is there a way to build a record from, say, a proplist in a way that doesn't require that I know all of the record's fields? For example:

P = [{type, "cow"}, {legs, 4}].

-record(animal, {type, legs}).

How can I create a new animal from the contents of P without doing:

#animal{type=..., legs=...}
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rvirding
Posted: Wed Nov 12, 2008 9:31 pm Reply with quote
User Joined: 30 Aug 2006 Posts: 452 Location: Stockholm, Sweden
Unfortunately no, there is no way to do that. You must remember that records are purely compile time constructions.
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keymone
Posted: Tue Jan 06, 2009 10:51 am Reply with quote
User Joined: 17 Sep 2007 Posts: 11 Location: Lviv, Ukraine
record is just a tuple, in your case: {animal, Type, Legs}

there is record_info/2 function which gives you a list of fields so you can do something like

Code:

build_record(record_name, Props) ->
  Fields = record_info(fields, record_name),
  build_record(record_name, Props, Fields).

build_record(record_name, Props, Fields) ->
  build_record(record_name, Props, Fields, {record_name}).

build_record(record_name, [], [], Record) ->
  {ok, Record};
build_record(record_name, Props, [], Record) ->
  {error, unknown_properties_in_proplist};
build_record(record_name, [{Field, Value}|Props], [Field|Fields], Record) ->
  NewRecord = append_element(Record, Value),
  build_record(record_name, Props, Fields, Record);
build_record(record_name, Props, [_|Fields], Record) ->
  NewRecord = append_element(Record, undefined),
  build_record(record_name, Props, Fields, Record).


but you must know at least record name, you can't pass it in variable

P.S. the above snippet of code has critical bug - if you provide a list of props in incorrect order record won't be created.
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