filter_foldl.erl

%% @doc
%% Filter module for Zotonic
%% 'foldl' filter, applies foldl to a list
%%
%% Usage:
%% {{ [1,2,3]|foldl:["*", 1] }}
%% {{ [1,2,3]|foldl:["+", 0] }}
%% {{ ["a", "b", "c"]|foldl:["++", ""] }}
%%
%% Pass an attribute to fetch for each item:
%% {{ user.o.shopping_basket|make_list|foldl:["price", "+", 0] }}
%% @end
-module(filter_foldl).

-export([
    foldl/3
]).

foldl(Items, Args, Context) ->
    case Args of 
        [Op, Acc] -> foldl_list(Items, [Op, Acc], Context);
        [Attr, Op, Acc] -> foldl_list_attr(Items, [Attr, Op, Acc], Context)
    end.
    
foldl_list(Items, [Op, Acc], _Context) ->
    lists:foldl(fun(Item, Acc1) -> 
        parse(Acc1, Op, Item)
    end, Acc, Items).
    
foldl_list_attr(Items, [Attr, Op, Acc], Context) ->
    lists:foldl(fun(Item, Acc1) -> 
        Prop = m_rsc:p(Item, Attr, Context),
        parse(Acc1, Op, Prop)
    end, Acc, Items).
    
parse(Val1, Op, Val2) ->
    Val1Str = z_convert:to_list(Val1),
    Val2Str = z_convert:to_list(Val2),
    Expr = case Op of
        "++" -> escape_string(Val1Str) ++ " " ++ Op ++ " " ++ escape_string(Val2Str);
        _ -> Val1Str ++ " " ++ Op ++ " " ++ Val2Str
    end,
    {ok, Tokens, _} = erl_scan:string(Expr ++ "."),
    {ok, Abstract} = erl_parse:parse_exprs(Tokens),
    Bindings = erl_eval:new_bindings(),
    {value, Value, _} = erl_eval:exprs(Abstract, Bindings),
    Value.

escape_string(V) -> "\"" ++ V ++ "\"".