exprecs.md

#Module exprecs#

• Description
• Data Types
• Function Index
• Function Details

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'([])}.

Compiling this (assuming exprecs is in the path) will produce the following code.

-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,
'#lens-'/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,
'#lens-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,
'#lens-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-'(any()) -> boolean().
'#is_record-'(X) ->
if
is_record(X, r) ->
true;
is_record(X, s) ->
true;
true ->
false
end.
-spec '#is_record-'(any(), any()) -> boolean().
'#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 '#lens-'('#prop-r'(), r) ->
{fun((#r{}) -> any()), fun((any(), #r{}) -> #r{})};
('#prop-s'(), s) ->
{fun((#s{}) -> any()), fun((any(), #s{}) -> #s{})}.
'#lens-'(Attr, r) ->
'#lens-r'(Attr);
'#lens-'(Attr, s) ->
'#lens-s'(Attr).
-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 '#lens-r'('#prop-r'()) ->
{fun((#r{}) -> any()), fun((any(), #r{}) -> #r{})}.
'#lens-r'(a) ->
{fun(R) ->
'#get-r'(a, R)
end,
fun(X, R) ->
'#set-r'([{a,X}], R)
end};
'#lens-r'(b) ->
{fun(R) ->
'#get-r'(b, R)
end,
fun(X, R) ->
'#set-r'([{b,X}], R)
end};
'#lens-r'(c) ->
{fun(R) ->
'#get-r'(c, R)
end,
fun(X, R) ->
'#set-r'([{c,X}], R)
end};
'#lens-r'(Attr) ->
error(bad_record_op, ['#lens-r',Attr]).
-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).
-spec '#lens-s'('#prop-s'()) ->
{fun((#s{}) -> any()), fun((any(), #s{}) -> #s{})}.
'#lens-s'(a) ->
{fun(R) ->
'#get-s'(a, R)
end,
fun(X, R) ->
'#set-s'([{a,X}], R)
end};
'#lens-s'(Attr) ->
error(bad_record_op, ['#lens-s', Attr]).
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

Creates a new record

get

Retrieves given attribute values from a record

set

Sets given attribute values in a record

fromlist

Creates a record from a key-value list

info

Equivalent to record_info/2

pos

Returns the position of a given attribute

is_record

Tests if a value is a specific record

convert

Converts an old record to the current version

prop

Used only in type specs

attr

Used only in type specs

lens

Returns a 'lens' (an accessor pair) as described in http://github.com/jlouis/erl-lenses

##Data Types##

###form()##

form() = any()

###forms()##

forms() = [form()]

###options()##

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

##Function Index##

parse_transform/2

##Function Details##

###parse_transform/2##

parse_transform(Forms::forms(), Options::options()) -> forms()