mad-cocktail/gin

The guards `in` and `beetween` for Erlang parse_transform

It is a tiny parse transform.

License: MIT

Author: Uvarov Michael (freeakk@gmail.com)

Guard in

This operator checks, that X is one of the [A, B, C, ...] values (Xs).

Xs is a fixed-size list of variables.

This operator and lists:member/2 are simular, but the second one is a function, when in is just an advanced macro command.

It is not possible to write using macros, because lists cannot be processed as a macro's argument.

This code is valid:

valid(X, Y) when in(X, [1,2,3,Y]) -> ok.

It will be replaced with:

valid(X, Y) when (X =:= 1 orelse X =:= 2 orelse X =:= 3 orelse X =:= Y) -> ok.

This code is invalid, because we don't know the actual size of the Y variable:

invalid(X, Y) when in(X, Y) -> error.

The numeric_in uses the == operator:

valid(X, Y) when numeric_in(X, [1,2,3,Y]) -> ok.

This code will be replaced with:

valid(X, Y) when (X == 1 orelse X == 2 orelse X == 3 orelse X == Y) -> ok.

The next code block demonstrates the difference beetween in and numeric_in:

> in(1, [1]).
true
> numeric_in(1, [1]).
true
> in(1, [1.0]).
false
> numeric_in(1, [1.0]).
true
> numeric_in({1}, [{1.0}]).
true

beetween operator

This is a realization of EEP-16.

S and E are stand for Start and End. X is a value to check.

beetween(X, S, E) checks, that X is inside the closed interval [S, E]. The S and E endpoints are included.

beetween(X, open(S), open(E)) checks, that X is inside the open interval (S, E).

beetween(X, open(S), E) checks, that X is inside the left-open interval (S, E].

beetween(X, S, open(E)) checks, that X is inside the right-open interval [S, E).

beetween(X, S, E) emulates the next macros:

-define(BEETWEEN(X, S, E), (((X) >= (S)) andalso ((X) =< (E))).

Example 1

Before:

...
 case ux_unidata_parser:split($;, Data) of
 [Code, Form, Props] when (Form=="NFC_QC" orelse Form=="NFKC_QC"
                    orelse Form=="NFD_QC" orelse Form=="NFKD_QC")
                      and (Props=="N"
                    orelse Props=="Y"
                    orelse Props=="M") ->
 Atom = list_to_atom(string:to_lower(Props)),
...

After:

 -compile({parse_transform, gin}).
...
 case ux_unidata_parser:split($;, Data) of
 [Code, Form, Props] when in(Form, ["NFC_QC", "NFKC_QC", "NFD_QC", "NFKD_QC"])
                      and in(Props, ["N", "Y", "M"]) ->
 Atom = list_to_atom(string:to_lower(Props)),
...

Example 2

Before:

maybe_change_depth(Depth, Char) when Char =:= $)
                                     orelse Char =:= $]
                                     orelse Char =:= $} ->
  Depth - 1;
maybe_change_depth(Depth, Char) when Char =:= $(
                                     orelse Char =:= $[
                                     orelse Char =:= ${ ->
  Depth + 1;
maybe_change_depth(Depth, _) ->
  Depth.

After:

 -compile({parse_transform, gin}).

maybe_change_depth(Depth, Char) when in(Char, ")]}") ->
  Depth - 1;
maybe_change_depth(Depth, Char) when in(Char, "([{") ->
  Depth + 1;
maybe_change_depth(Depth, _) ->
  Depth.

Example 3

From here.

Before:

type_cast_row_data(Data, #field{type=Type})
when Type == ?FIELD_TYPE_VARCHAR;
Type == ?FIELD_TYPE_TINY_BLOB;
Type == ?FIELD_TYPE_MEDIUM_BLOB;
Type == ?FIELD_TYPE_LONG_BLOB;
Type == ?FIELD_TYPE_BLOB;
Type == ?FIELD_TYPE_VAR_STRING;
Type == ?FIELD_TYPE_STRING ->
Data;

type_cast_row_data(Data, #field{type=Type})
when Type == ?FIELD_TYPE_TINY;
Type == ?FIELD_TYPE_SHORT;
Type == ?FIELD_TYPE_LONG;
Type == ?FIELD_TYPE_LONGLONG;
Type == ?FIELD_TYPE_INT24;
Type == ?FIELD_TYPE_YEAR ->
list_to_integer(binary_to_list(Data));

...

Type cannot be a floated value, use =:= for comparation.

After:

type_cast_row_data(Data, #field{type=Type}) when
    in(Type, [?FIELD_TYPE_VARCHAR,      ?FIELD_TYPE_TINY_BLOB,
              ?FIELD_TYPE_MEDIUM_BLOB,  ?FIELD_TYPE_LONG_BLOB,
              ?FIELD_TYPE_BLOB,         ?FIELD_TYPE_VAR_STRING,
              ?FIELD_TYPE_STRING]) ->
Data;

type_cast_row_data(Data, #field{type=Type}) when
    in(Type, [?FIELD_TYPE_TINY,     ?FIELD_TYPE_SHORT,  ?FIELD_TYPE_LONG,
              ?FIELD_TYPE_LONGLONG, ?FIELD_TYPE_INT24,  ?FIELD_TYPE_YEAR] ->
list_to_integer(binary_to_list(Data));

...

Example 4

Before:

otp_7198_scan(<<>>, TokAcc) ->
        lists:reverse(['$thats_all_folks$' | TokAcc]);

otp_7198_scan(<<D, Z, Rest/binary>>, TokAcc) when
                        (D =:= $D orelse D =:= $d) and
                        ((Z =:= $\s) or (Z =:= $() or (Z =:= $))) ->
        otp_7198_scan(<<Z, Rest/binary>>, ['AND' | TokAcc]);

otp_7198_scan(<<D>>, TokAcc) when
                        (D =:= $D) or (D =:= $d) ->
        otp_7198_scan(<<>>, ['AND' | TokAcc]);

otp_7198_scan(<<N, Z, Rest/binary>>, TokAcc) when
                        (N =:= $N orelse N =:= $n) and
                        ((Z =:= $\s) or (Z =:= $() or (Z =:= $))) ->
        otp_7198_scan(<<Z, Rest/binary>>, ['NOT' | TokAcc]);

otp_7198_scan(<<C, Rest/binary>>, TokAcc) when
                                (C >= $A) and (C =< $Z);
                                (C >= $a) and (C =< $z);
                                (C >= $0) and (C =< $9) ->
        case Rest of
                <<$:, R/binary>> ->
                        otp_7198_scan(R, [{'FIELD', C} | TokAcc]);
                _ ->
                        otp_7198_scan(Rest, [{'KEYWORD', C} | TokAcc])
        end.
...

After:

 -compile({parse_transform, gin}).

otp_7198_scan(<<>>, TokAcc) ->
    lists:reverse(['$thats_all_folks$' | TokAcc]);

otp_7198_scan(<<D, Z, Rest/binary>>, TokAcc)
    when in(D, "Dd") and in(Z, "\s()") ->
    otp_7198_scan(<<Z, Rest/binary>>, ['AND' | TokAcc]);

otp_7198_scan(<<D>>, TokAcc) when in(D, "Dd") ->
    otp_7198_scan(<<>>, ['AND' | TokAcc]);

otp_7198_scan(<<N, Z, Rest/binary>>, TokAcc)
    when in(N, "Nn") and in(Z, "\s()") ->
    otp_7198_scan(<<Z, Rest/binary>>, ['NOT' | TokAcc]);

otp_7198_scan(<<C, Rest/binary>>, TokAcc)
    when beetween(C, $A, $Z); beetween(C, $a, $z); beetween(C, $0, $9) ->
    case Rest of
        <<$:, R/binary>> ->
            otp_7198_scan(R, [{'FIELD', C} | TokAcc]);
        _ ->
            otp_7198_scan(Rest, [{'KEYWORD', C} | TokAcc])
    end.

...