As a mental break and to support my erlang version of Cucumber (“BravoDelta”), I spent a little time building out a JSON parser.
Some notes before the code:
- I could have done it without the case statements (using pattern matching in the functions) but I felt the code was not as readable and there were some cases where I needed the RAW input.
- I used records because it was important to return BOTH the list and the remaining text. It also improves the readability if you follow know the syntax (#json = new record, JSON#json = existing)
- Has minimal error checking – fails = good in a BDD tool
- Assumed that keys are “safe” words (don’t really need quotes)
Here’s the code. Enjoy!
Note 2013-11-15: Here’s the active source for this on github.
-export([json/1]).
-record(json, {list=[], raw=[]}).
-record(jsonkv, {value=[], raw=[]}).
% handles values that are quoted (this one ends the quote)
json_value_quoted(Value, [$" | T]) ->
#jsonkv{value=Value, raw=T};
json_value_quoted(Value, [Next | T]) ->
json_value_quoted(Value ++ [Next], T).
% returns JSON Key Values with remaining JSON
json_value(Value, RawJSON) ->
[Next | T] = RawJSON,
case Next of
$: -> throw('unexpected token');
${ -> J = json(RawJSON), % recurse to get list
#jsonkv{value=J#json.list, raw=J#json.raw};
$, -> #jsonkv{value=string:strip(Value), raw=RawJSON}; % terminator, return
$} -> #jsonkv{value=string:strip(Value), raw=RawJSON}; % terminator, return
$" -> json_value_quoted(Value, T); % run to next quote,exit
_ -> json_value(Value ++ [Next], T) % recurse
end.
% parses the Key Value pairs (KVPs) based on , & } delimiters
json(JSON, Key) ->
[Next | T] = JSON#json.raw,
case {Next, T} of
{$", _} -> json(JSON#json{raw=T}, Key); % ignore
{${, _} -> json(#json{raw=T}, []); % start new hash
{$,, _} -> json(JSON#json{raw=T}, []); % add new value
{$:, _} -> KV = json_value([], T), % get value for key
List = lists:merge(JSON#json.list, [{string:strip(Key), KV#jsonkv.value}]),
json(#json{list=List, raw=KV#jsonkv.raw}, []); % add new KVP
{$}, []} -> JSON#json.list; %DONE!
{$}, _} -> JSON#json{raw=T}; %List parse, but more remains!
{_, _} -> json(JSON#json{raw=T}, Key ++ [Next]) % add to key
end.
% entry point
json(RawJSON) ->
json(#json{raw=RawJSON}, []).
Rob Hirschfeld 