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Module exprecs

Parse transform for generating record access functions.

Authors: : Ulf Wiger (ulf.wiger@ericsson.com).

Description

This parse transform can be used to reduce compile-time dependencies in large systems.

In the old days, before records, Erlang programmers often wrote access functions for tuple data. This was tedious and error-prone. The record syntax made this easier, but since records were implemented fully in the pre-processor, a nasty compile-time dependency was introduced.

This module automates the generation of access functions for records. While this method cannot fully replace the utility of pattern matching, it does allow a fair bit of functionality on records without the need for compile-time dependencies.

Whenever record definitions need to be exported from a module, inserting a compiler attribute, export_records([RecName|...]) causes this transform to lay out access functions for the exported records:

As an example, consider the following module:

  -module(test_exprecs).
  -export([f/0]).
 
  -compile({parse_transform, exprecs}).
 
  -record(r, {a = 0 :: integer(),
              b = 0 :: integer(),
              c = 0 :: integer()}).
 
  -record(s,{a}).
 
  -export_records([r,s]).
 
  f() ->
      {new,'#new-r'([])}.
  
  -module(test_exprecs).
  -compile({pt_pp_src,true}).
  -export([f/0]).
  -record(r,{a = 0 :: integer(),b = 0 :: integer(),c = 0 :: integer()}).
  -record(s,{a}).
  -export_records([r,s]).
  -export(['#exported_records-'/0,
           '#new-'/1,
           '#info-'/1,
           '#info-'/2,
           '#pos-'/2,
           '#is_record-'/1,
           '#is_record-'/2,
           '#get-'/2,
           '#set-'/2,
           '#fromlist-'/2,
           '#new-r'/0,
           '#new-r'/1,
           '#get-r'/2,
           '#set-r'/2,
           '#pos-r'/1,
           '#fromlist-r'/1,
           '#fromlist-r'/2,
           '#info-r'/1,
           '#new-s'/0,
           '#new-s'/1,
           '#get-s'/2,
           '#set-s'/2,
           '#pos-s'/1,
           '#fromlist-s'/1,
           '#fromlist-s'/2,
           '#info-s'/1]).
 
  -type '#prop-r'() :: {a, integer()} | {b, integer()} | {c, integer()}.
  -type '#attr-r'() :: a | b | c.
  -type '#prop-s'() :: {a, any()}.
  -type '#attr-s'() :: a.
 
  -spec '#exported_records-'() -> [r | s].
  '#exported_records-'() ->
      [r,s].
 
  -spec '#new-'(r) -> #r{};
               (s) -> #s{}.
  '#new-'(r) ->
      '#new-r'();
  '#new-'(s) ->
      '#new-s'().
 
  -spec '#info-'(r) -> [a | b | c];
                (s) -> [a].
  '#info-'(RecName) ->
      '#info-'(RecName, fields).
 
  -spec '#info-'(r, size) -> 4;
                (r, fields) -> [a | b | c];
                (s, size) -> 2;
                (s, fields) -> [a].
  '#info-'(r, Info) ->
      '#info-r'(Info);
  '#info-'(s, Info) ->
      '#info-s'(Info).
 
  -spec '#pos-'(r, a) -> 1;
               (r, b) -> 2;
               (r, c) -> 3;
               (s, a) -> 1.
  '#pos-'(r, Attr) ->
      '#pos-r'(Attr);
  '#pos-'(s, Attr) ->
      '#pos-s'(Attr).
 
  -spec '#is_record-'(#r{}) -> true;
                     (#s{}) -> true;
                     (any()) -> false.
  '#is_record-'(X) ->
      if
          is_record(X, r) ->
              true;
          is_record(X, s) ->
              true;
          true ->
              false
      end.
 
  -spec '#is_record-'(r, #r{}) -> true;
                     (s, #s{}) -> true;
                     (any(), any()) -> false.
  '#is_record-'(s, Rec) when tuple_size(Rec) == 2, element(1, Rec) == s ->
      true;
  '#is_record-'(r, Rec) when tuple_size(Rec) == 4, element(1, Rec) == r ->
      true;
  '#is_record-'(_, _) ->
      false.
 
  -spec '#get-'(a, #r{}) -> integer();
               (b, #r{}) -> integer();
               (c, #r{}) -> integer();
               (a, #s{}) -> any();
               (['#attr-r'()], #r{}) -> [integer()];
               (['#attr-s'()], #s{}) -> [any()].
  '#get-'(Attrs, Rec) when is_record(Rec, r) ->
      '#get-r'(Attrs, Rec);
  '#get-'(Attrs, Rec) when is_record(Rec, s) ->
      '#get-s'(Attrs, Rec).
 
  -spec '#set-'(['#prop-r'()], #r{}) -> #r{};
               (['#prop-s'()], #s{}) -> #s{}.
  '#set-'(Vals, Rec) when is_record(Rec, r) ->
      '#set-r'(Vals, Rec);
  '#set-'(Vals, Rec) when is_record(Rec, s) ->
      '#set-s'(Vals, Rec).
 
  -spec '#fromlist-'(['#prop-r'()], #r{}) -> #r{};
                    (['#prop-s'()], #s{}) -> #s{}.
  '#fromlist-'(Vals, Rec) when is_record(Rec, r) ->
      '#fromlist-r'(Vals, Rec);
  '#fromlist-'(Vals, Rec) when is_record(Rec, s) ->
      '#fromlist-s'(Vals, Rec).
 
  -spec '#new-r'() -> #r{}.
  '#new-r'() ->
      #r{}.
 
  -spec '#new-r'(['#prop-r'()]) -> #r{}.
  '#new-r'(Vals) ->
      '#set-r'(Vals, #r{}).
 
  -spec '#get-r'(a, #r{}) -> integer();
                (b, #r{}) -> integer();
                (c, #r{}) -> integer();
                (['#attr-r'()], #r{}) -> [integer()].
  '#get-r'(Attrs, R) when is_list(Attrs) ->
      [
       '#get-r'(A, R) ||
           A <- Attrs
      ];
  '#get-r'(a, R) ->
      R#r.a;
  '#get-r'(b, R) ->
      R#r.b;
  '#get-r'(c, R) ->
      R#r.c;
  '#get-r'(Attr, R) ->
      error(bad_record_op, ['#get-r',Attr,R]).
 
  -spec '#set-r'(['#prop-r'()], #r{}) -> #r{}.
  '#set-r'(Vals, Rec) ->
      F = fun([], R, _F1) ->
                 R;
             ([{a,V}|T], R, F1) when is_list(T) ->
                 F1(T, R#r{a = V}, F1);
             ([{b,V}|T], R, F1) when is_list(T) ->
                 F1(T, R#r{b = V}, F1);
             ([{c,V}|T], R, F1) when is_list(T) ->
                 F1(T, R#r{c = V}, F1);
             (Vs, R, _) ->
                 error(bad_record_op, ['#set-r',Vs,R])
          end,
      F(Vals, Rec, F).
 
  -spec '#fromlist-r'(['#prop-r'()]) -> #r{}.
  '#fromlist-r'(Vals) when is_list(Vals) ->
      '#fromlist-r'(Vals, '#new-r'()).
 
  -spec '#fromlist-r'(['#prop-r'()], #r{}) -> #r{}.
  '#fromlist-r'(Vals, Rec) ->
      AttrNames = [{a,2},{b,3},{c,4}],
      F = fun([], R, _F1) ->
                 R;
             ([{H,Pos}|T], R, F1) when is_list(T) ->
                 case lists:keyfind(H, 1, Vals) of
                     false ->
                         F1(T, R, F1);
                     {_,Val} ->
                         F1(T, setelement(Pos, R, Val), F1)
                 end
          end,
      F(AttrNames, Rec, F).
 
  -spec '#pos-r'('#attr-r'() | atom()) -> integer().
  '#pos-r'(a) ->
      2;
  '#pos-r'(b) ->
      3;
  '#pos-r'(c) ->
      4;
  '#pos-r'(A) when is_atom(A) ->
      0.
 
  -spec '#info-r'(fields) -> [a | b | c];
                 (size) -> 3.
  '#info-r'(fields) ->
      record_info(fields, r);
  '#info-r'(size) ->
      record_info(size, r).
 
  -spec '#new-s'() -> #s{}.
  '#new-s'() ->
      #s{}.
 
  -spec '#new-s'(['#prop-s'()]) -> #s{}.
  '#new-s'(Vals) ->
      '#set-s'(Vals, #s{}).
 
  -spec '#get-s'(a, #s{}) -> any();
                (['#attr-s'()], #s{}) -> [any()].
  '#get-s'(Attrs, R) when is_list(Attrs) ->
      [
       '#get-s'(A, R) ||
           A <- Attrs
      ];
  '#get-s'(a, R) ->
      R#s.a;
  '#get-s'(Attr, R) ->
      error(bad_record_op, ['#get-s',Attr,R]).
 
  -spec '#set-s'(['#prop-s'()], #s{}) -> #s{}.
  '#set-s'(Vals, Rec) ->
      F = fun([], R, _F1) ->
                 R;
             ([{a,V}|T], R, F1) when is_list(T) ->
                 F1(T, R#s{a = V}, F1);
             (Vs, R, _) ->
                 error(bad_record_op, ['#set-s',Vs,R])
          end,
      F(Vals, Rec, F).
 
  -spec '#fromlist-s'(['#prop-s'()]) -> #s{}.
  '#fromlist-s'(Vals) when is_list(Vals) ->
      '#fromlist-s'(Vals, '#new-s'()).
 
  -spec '#fromlist-s'(['#prop-s'()], #s{}) -> #s{}.
  '#fromlist-s'(Vals, Rec) ->
      AttrNames = [{a,2}],
      F = fun([], R, _F1) ->
                 R;
             ([{H,Pos}|T], R, F1) when is_list(T) ->
                 case lists:keyfind(H, 1, Vals) of
                     false ->
                         F1(T, R, F1);
                     {_,Val} ->
                         F1(T, setelement(Pos, R, Val), F1)
                 end
          end,
      F(AttrNames, Rec, F).
 
  -spec '#pos-s'('#attr-s'() | atom()) -> integer().
  '#pos-s'(a) ->
      2;
  '#pos-s'(A) when is_atom(A) ->
      0.
 
  -spec '#info-s'(fields) -> [a];
                 (size) -> 1.
  '#info-s'(fields) ->
      record_info(fields, s);
  '#info-s'(size) ->
      record_info(size, s).
 
  f() ->
      {new,'#new-r'([])}.
 
  

It is possible to modify the naming rules of exprecs, through the use
of the following attributes (example reflecting the current rules):

  -exprecs_prefix(["#", operation, "-"]).
  -exprecs_fname([prefix, record]).
  -exprecs_vfname([fname, "__", version]).
  

The lists must contain strings or any of the following control atoms:

  • in exprecs_prefix: operation

  • in exprecs_fname: operation, record, prefix

  • in exprecs_vfname: operation, record, prefix, fname, version

Exprecs will substitute the control atoms with the string values of the
corresponding items. The result will then be flattened and converted to an
atom (a valid function or type name).

operation is one of:

  • new

  • get

  • set

  • fromlist

  • info

  • pos

  • is_record

  • convert

  • prop

  • attr

Data Types

form()

form() = any()

forms()

forms() = [[form()](#type-form)]

options()

options() = [{atom(), any()}]

Function Index

parse_transform/2

Function Details

parse_transform/2

parse_transform(Forms::[forms()](#type-forms), Options::[options()](#type-options)) -> [forms()](#type-forms)



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