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Destructuring JSON validator/converter for Erlang
Erlang Makefile
Branch: master



Valijate is a data validator and canonicalizer written in Erlang. Valijate combines the tasks of validating that a value has a given type, and putting the data therein on a form which makes it easy to process with Erlang’s pattern matching.

Valijate can handle Erlang terms and JSON values. The JSON value formats emitted by the mochijson2 and ejson JSON parsers are supported.

Comparison to similar tools

There is a language-independent JSON type language, “JSON-schema”. Jesse is an Erlang implementation of a JSON-schema validator. Valijate is more lightweight, ad-hoc and Erlang-specific.

The ej set of Erlang JSON helper functions also contains a JSON validator. Valijate is different in that instead of providing some functions for validating and others for accessing JSON values, it combines those two tasks.

Also, it support validation of Erlang terms – for instance, validation and canonicalization of application config settings.


See doc/ for a detailed API description.

For examples of its use, read on.



The following example converts a JSON string into an Erlang term of the form {Authors, Title, Available, Price}.

> S = "{\"title\": \"Hogwarts, a History\", \"authors\":[\"Bathilda Bagshot\"], "++
      " \"price\": 3.99, \"available\": true}".
> T = {object, [{<<"authors">>, {array,string}}, {<<"title">>, string},
                {<<"available">>, boolean}, {<<"price">>, number}]}.
> J = mochijson2:decode(S).
> {ok,V} = valijate:json(J,T).

=> {ok,{[<<"Bathilda Bagshot">>], <<"Hogwarts, a History">>,true,3.99}}

Note that the type specification determines the order of values in the resulting tuple - the order within the JSON object does not matter.


The following example validates an Erlang term (here a key-value list) and converts it into the canonicalized form {Authors, Title, Available, Price}.

   > E = [{title, "Hogwarts, a History"}, {authors, ["Bathilda Bagshot"]},
          {price, 3.99}, {available, true}].
   > T = {proplist, [{authors, {list, string}}, {title, string},
                     {available, boolean}, {price, number}]}.
   > {ok,V} = valijate:erlang(E,T).
=> {ok,{["Bathilda Bagshot"],"Hogwarts, a History",true,3.99}}

Note that the type specification determines the order of values in the resulting tuple - the order within the property list does not matter.

Erlang configuration:

One usecase for applying validation is for reading configuration settings. Valijate includes a convenient shortcut for that purpose.

Here’s a simple example – reading the kernel parameter ‘net_ticktime’:

> valijate:get_env(kernel, net_ticktime, {default, 60}, integer).

If the validation rule is simple, so it the code to read the parameter.

Another example is the kernel parameter ‘dist_auto_connect’ which may have the values ‘never’ ‘once’, or be absent, in which case the meaning is “always auto-connect”. That parameter can be validated and read like this:

> valijate:get_env(kernel, dist_auto_connect,
                   {member, [never,once]}).
=> {ok,once} or {ok,always} or {ok,never}

For a more complex example, take the following kernel parameter:

“net_setuptime = SetupTime:

SetupTime must be a positive integer or floating point number, and will be interpreted as the maximally allowed time for each network operation during connection setup to another Erlang node. The maxi‐ mum allowed value is 120; if higher values are given, 120 will be used. The default value if the variable is not given, or if the value is incorrect (e.g. not a number), is 7 seconds.”

There are several things going on, but it can all be expressed in a Valijate type expression. The parameter can be validated and read like this:

> application:set_env(kernel, net_setuptime, 175).
> valijate:get_env(kernel, net_setuptime,
                     {satisfy, fun(V) -> V>0 end, "must be positive"},
                     {convert, fun(V) -> {ok,min(V,120)} end, "roof of 120"}]}).
=> {ok,120}

Error handling

A value can fail validation in several different ways. In all cases, the validator will report the location of the troubling item. Regardless of what kind of value it was (Erlang term or JSON) which failed validation, the function valijate:error_to_english/1 can be used to obtain a human-readable description of the error (in English).

Wrong type (Erlang / JSON)

The simplest kind of error is when an item is expected to have one type, but turns out to have another type:

> E1 = valijate:json(mochijson2:decode("[]"), number).
=> {validation_error,[],{wrong_type,[],array,number}}

> valijate:error_to_english(E1).
=> "At path <root> : Value has type array, but number was expected"

The path to the error is reported. For JSON array items, the path is the (zero-based) index of the item:

> E2 = valijate:json(mochijson2:decode("[true, false, 42]"),
                     {array, boolean}).
> valijate:error_to_english(E2).
=> "At path [2] : Value 42 has type number, but boolean was expected"

For JSON objects, the path is the name of the object field:

> E3 = valijate:json(mochijson2:decode("{\"foo\":\"42\"}"),
       {object, [{<<"foo">>, number}]}).
> valijate:error_to_english(E3).
=> "At path .foo : Value has type string, but number was expected"

> E4 = valijate:json(mochijson2:decode("{\"foo\": {\"bar\":\"42\"}}"),
       {object, [{<<"foo">>, {object, [{<<"bar">>, number}]}}]}).
> valijate:error_to_english(E4).
=> "At path : Value has type string, but number was expected"

Similarly for Erlang property lists:

> E4 = valijate:erlang([{foo, [{bar,"42"}]}],
                       {proplist, [{foo, {proplist, [{bar,integer}]}}]}).
> valijate:error_to_english(E4).
=> "At path : Value has type list, but integer was expected"

Missing object field (JSON)

Object fields may be required or optional. If a required field is missing, it is reported as an error:

> E10 = valijate:json(mochijson2:decode("{}"),
                      {object, [{<<"heavy">>, boolean}]}).
> valijate:error_to_english(E10).
=> "At path <root> : The object is missing field \"heavy\""

Optional fields may be absent, in which case their default value is filled in:

> valijate:json(mochijson2:decode("{}"),
                {object, [{opt, <<"heavy">>, boolean, maybe}]}).
=> {ok,{maybe}}

Superfluous object fields (JSON)

As default, objects may not contain fields which are not mentioned in the type specification. Extra fields are reported as an error:

> E12 = valijate:json(mochijson2:decode("{\"a\":12, \"b\":23, \"c\":34}"),
                      {object, [{<<"b">>, number}]}).
> valijate:error_to_english(E12).
=> "At path <root> : The object has superfluous fields: \"a\", \"c\""

If desired, extra fields may be caught and preserved:

> valijate:json(mochijson2:decode("{\"a\":12, \"b\":23, \"c\":34}"),
                {object, [{<<"b">>, number},
                              {keep_rest, fun(X) -> {rest, X} end}]}).
=> {ok,{23,{rest,[{<<"a">>,12},{<<"c">>,34}]}}}

Constraint not satisfied

Items can be subject to custom constraints:

> IsPhoneNo = fun(X) -> re:run(X, "^[0-9]+$", [{capture, none}]) == match end.
> valijate:json(mochijson2:decode("{\"phone\": \"12349\"}"),
                {object, [{<<"phone">>, {satisfy, IsPhoneNo, "phone number"}}]}).
=> {ok,{<<"12349">>}}

> E15 = valijate:json(mochijson2:decode("{\"phone\": \"+555\"}"),
                      {object, [{<<"phone">>, {satisfy, IsPhoneNo, "phone number"}}]}).
> valijate:error_to_english(E15).
=> "At path .phone : The value does not satisfy phone number: <<\"+555\">>\n"
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