# How to evaluate expressions containing record reference? **URL:** \ **Category:** Questions / Help\ **Tags:** erlang\ **Created:** [February 7, 2024, 4:03pm UTC](https://erlangforums.com/t/how-to-evaluate-expressions-containing-record-reference/3273 "2024-02-07T16:03:50Z")\ **Posts on this page:** 7\ **Page:** 1
**Author:** ![etnt](https://erlangforums.com/user_avatar/erlangforums.com/etnt/32/892_2.png) [@etnt](https://erlangforums.com/u/etnt)\ **Post date:** [February 7, 2024, 4:03pm UTC](https://erlangforums.com/t/how-to-evaluate-expressions-containing-record-reference/3273/1 "2024-02-07T16:03:50Z")
Hi, How can I evaluate expressions such as: “X = P#person.age.” using something like erl\_eval:exprs and friends? Sort of like this (non-working code): ```erlang my_eval() -> PersonDef = {attribute,38,record, {person,[{record_field,38,{atom,38,name}}, {record_field,38,{atom,38,age}}]}}, RecordDefs = [{person, 38, PersonDef}], Person = #person{name = "John", age = 30}, Bindings0 = erl_eval:new_bindings(), Bindings = erl_eval:add_binding('P', Person, Bindings0), eval_exprs("X = P#person.age.", Bindings, RecordDefs). eval_exprs(Expression, Bindings, RecordDefs) -> %% Parse the expression {ok, Tokens, _} = erl_scan:string(Expression), {ok, [Expr]} = erl_parse:parse_exprs(Tokens), %% Evaluate the expression case erl_eval:exprs([Expr], Bindings, RecordDefs) of {value, Value, NewBindings} -> {Value, NewBindings}; Error -> Error end. ``` ---
**Author:** ![mmin](https://erlangforums.com/user_avatar/erlangforums.com/mmin/32/2193_2.png) [@mmin](https://erlangforums.com/u/mmin)\ **Post date:** [February 7, 2024, 4:22pm UTC](https://erlangforums.com/t/how-to-evaluate-expressions-containing-record-reference/3273/2 "2024-02-07T16:22:53Z")
I don’t think that’s possible, but what is possible is to use [erl\_expand\_records](https://www.erlang.org/doc/man/erl_expand_records#module-2) module to expand records to tuples. It will return modified `abstract_form()`s and you can act like that is what you originally parsed. ---
**Author:** ![williamthome](https://erlangforums.com/user_avatar/erlangforums.com/williamthome/32/471_2.png) [@williamthome](https://erlangforums.com/u/williamthome)\ **Post date:** [February 7, 2024, 4:46pm UTC](https://erlangforums.com/t/how-to-evaluate-expressions-containing-record-reference/3273/3 "2024-02-07T16:46:48Z")
I believe there is no easy way to achieve that. Maybe you can try [erl\_eval\_records](https://github.com/petrkozorezov/erl_eval_records/tree/master) lib. ---
**Author:** ![williamthome](https://erlangforums.com/user_avatar/erlangforums.com/williamthome/32/471_2.png) [@williamthome](https://erlangforums.com/u/williamthome)\ **Post date:** [February 7, 2024, 5:04pm UTC](https://erlangforums.com/t/how-to-evaluate-expressions-containing-record-reference/3273/4 "2024-02-07T17:04:04Z")
Or change your code to call a function ```erlang -module(my_person). -export([get_person_age/1, my_eval/0]). -record(person, {name, age}). get_person_age(#person{age = Age}) -> Age. my_eval() -> Person = #person{name = "John", age = 30}, Bindings0 = erl_eval:new_bindings(), Bindings = erl_eval:add_binding('P', Person, Bindings0), eval_exprs("X = my_person:get_person_age(P).", Bindings). %% <-- Uses a function eval_exprs(Expression, Bindings) -> %% Parse the expression {ok, Tokens, _} = erl_scan:string(Expression), {ok, [Expr]} = erl_parse:parse_exprs(Tokens), %% Evaluate the expression case erl_eval:exprs([Expr], Bindings) of {value, Value, NewBindings} -> {Value, NewBindings}; Error -> Error end. ``` Result ```erlang 1> my_person:my_eval(). {30,[{'P',{person,"John",30}},{'X',30}]} ``` ---
**Author:** ![etnt](https://erlangforums.com/user_avatar/erlangforums.com/etnt/32/892_2.png) [@etnt](https://erlangforums.com/u/etnt)\ **Post date:** [February 9, 2024, 10:52am UTC](https://erlangforums.com/t/how-to-evaluate-expressions-containing-record-reference/3273/5 "2024-02-09T10:52:09Z")
Mm…thanks for the suggestions. A pity that eval\_exprs/N can’t handle it, perhaps I could wish for a feature enhancement in some future release… 😛 ---
**Author:** ![williamthome](https://erlangforums.com/user_avatar/erlangforums.com/williamthome/32/471_2.png) [@williamthome](https://erlangforums.com/u/williamthome)\ **Post date:** [February 9, 2024, 10:20pm UTC](https://erlangforums.com/t/how-to-evaluate-expressions-containing-record-reference/3273/6 "2024-02-09T22:20:24Z")
Using the `erl_expand_records` that @mmin mentioned, I’ve ended up doing something that can maybe help you. `erl_expand_records` expects record attributes, and what it expands are functions, so we need to wrap the parsed forms into a function. Take this (tricky) example (it needs to be improved using `erl_syntax`): ```erlang -module(eval). -export([string/3, record_attribute/2]). string(String, Bindings, HeaderForms) -> ExprForms = wrap(expr_forms(String)), Forms = lists:last(erl_expand_records:module(HeaderForms ++ ExprForms, [])), erl_eval:expr(unwrap(Forms), Bindings). record_attribute(Name, Fields0) when is_atom(Name), is_list(Fields0) -> Fields1 = [atom_to_binary(Field) || Field <- Fields0], Fields = lists:join(", ", Fields1), IOList = io_lib:format("-record(~s, {~s}).", [Name, Fields]), merl:quote(iolist_to_binary(IOList)). %% Internal functions expr_forms(String) -> {ok, Tokens, _} = erl_scan:string(String), {ok, Forms} = erl_parse:parse_exprs(Tokens), Forms. wrap(Expr) -> [{function, 1, {atom, 1, foo}, 1, [{clause, 1, [], [], Expr}]}]. unwrap({function, 1, {atom, 1, foo}, 1, [{clause, 1, [], [], [Expr]}]}) -> Expr. ``` Result: ```erlang 1> HeaderForms = [eval:record_attribute(person, [name, age])]. [{attribute,1,record, {person,[{record_field,1,{atom,1,name}}, {record_field,1,{atom,1,age}}]}}] 2> eval:string("X = P#person.age.", #{'P' => {person, "John", 30}}, HeaderForms). {value,30,#{'X' => 30,'P' => {person,"John",30},rec0 => 30}} ``` Hope it can help you. ---
**Author:** ![etnt](https://erlangforums.com/user_avatar/erlangforums.com/etnt/32/892_2.png) [@etnt](https://erlangforums.com/u/etnt)\ **Post date:** [February 12, 2024, 10:41pm UTC](https://erlangforums.com/t/how-to-evaluate-expressions-containing-record-reference/3273/7 "2024-02-12T22:41:42Z")
Thanks! I’ll have a look.