guile-json

guile-json is a JSON module for Guile. It supports parsing and building JSON documents according to the http://json.org specification.

• Complies with http://json.org specification.

• Builds JSON documents programmatically using scheme data types.

• Allows JSON pretty printing.

Installation

Download the latest tarball and untar it:

• guile-json-4.5.2.tar.gz

If you are cloning the repository make sure you run this first:

$ autoreconf -vif

Then, run the typical sequence:

$ ./configure --prefix=<guile-prefix>
$ make
$ sudo make install

Where should preferably be the same as your system Guile installation directory (e.g. /usr).

If everything installed successfully you should be up and running:

$ guile
[email protected](guile-user)> (use-modules (json))
[email protected](guile-user)> (scm->json #(1 2 3))
[1,2,3]

It might be that you installed guile-json somewhere differently than your system's Guile. If so, you need to indicate Guile where to find guile-json, for example:

$ GUILE_LOAD_PATH=/usr/local/share/guile/site guile

Usage

guile-json provides a few procedures to parse and build a JSON document. A JSON document is transformed into or from native Guile values according to the following table:

Why are JSON arrays converted to vectors and JSON objects to alists? See this discussion for details.

By default the value of JSON "null" is mapped to the symbol 'null. However, all guile-json functions allow changing the default null value by specifying the #:null keyword argument with another value. This other value needs to be recognized by eq?.

To start using guile-json procedures and macros you first need to load the module:

[email protected](guile-user)> (use-modules (json))

Procedures

• (json->scm #:optional port #:key null) : Reads a JSON document from the given port, or from the current input port if none is given.

  Optional arguments:

  • port : is optional, it defaults to the current input port.

  Keyword arguments:

  • null : value for JSON's null, it defaults to the 'null symbol.

• (json-string->scm str #:key null) : Reads a JSON document from the given string.

  Keyword arguments:

  • null : value for JSON's null, it defaults to the 'null symbol.

• (scm->json native #:optional port #:key solidus unicode null validate pretty) : Creates a JSON document from the given native Guile value. The JSON document is written into the given port, or to the current output port if non is given.

  Optional arguments:

  • port : it defaults to the current output port.

  Keyword arguments:

  • solidus : if true, the slash (/ solidus) character will be escaped (defaults to false).

  • unicode : if true, additional to control characters, non-ASCII characters will be escaped as well (defaults to false).

  • null : value for JSON's null (defaults to the 'null symbol).

  • validate : if true, the native value will be validated before starting to print the JSON document (defaults to true).

  • pretty : if true, the JSON document will be pretty printed (defaults to false).

• (scm->json-string native #:key solidus unicode null validate pretty) : Creates a JSON document from the given native Guile value into a string. For keyword arguments meaning see scm->json.

  Note that when using alists to build JSON objects, symbols or numbers might be used as keys and they both will be converted to strings.

Exceptions

A json-invalid exception is thrown if an error is found during the JSON parsing with a single port argument. The line or column where the error occured can be easily obtained from the port by calling port-line or port-column.

When building a JSON document from a native type a json-invalid exception might be thrown with the offending value as an argument (see table above for supported types).

JSON Objects and Records

guile-json 4.5.0 introduces JSON types, a new feature that allows converting JSON objects into record types and vice versa in a very straight forward way. This was built on top of define-json-mapping which was introduced in version 4.2.0.

Let's take a look at an example. Imagine we have the following user account information:

{
  "id": 1234,
  "username": "jane"
}

We can easily create a record representing that data with define-json-type by simply doing:

> (define-json-type <account>
    (id)
    (username))

This will define the record constructor, the predicate and conversion procedures like json->account or account->json (see define-json-type for more details).

We can now create a new account and check its contents as with regular records:

> (define account (make-account "1234" "jane"))
> (account-id account)
"1234"
> (account-username account)
"jane"

Or we can use the auto-generated scm->account to create the account:

> (define account (scm->account '(("id" . "1234") ("username" . "jane"))))

It is also possible to convert the record to a JSON string:

> (account->json account)
"{\"id\":\"1234\",\"username\":\"jane\"}"

Or from a JSON string to a new record:

> (define json-account "{\"id\":\"1234\",\"username\":\"jane\"}")
> (json->account json-account)
#<<account> id: "1234" username: "jane">

Macros

• (define-json-type rtd (field key type) ...) : Define a new mapping between a JSON object and a record type. This will automatically define the record constructor, the predicate, all field getters and record to/from JSON conversions. For more control use define-json-mapping.

  • rtd : the name of the record type.

  • ((field key type) ...) : a series of field specifications.

    • field : the name of a JSON object field.

    • key : a different name for the field of this JSON object. If given, this name will be used instead of the field name when serializing or deserializing.

    • type : indicates that this field contains a record type. It is also possible to indicate that the field contains an array of objects of the same record type by using the vector syntax #(type).

• (define-json-mapping rtd ctor pred json->record [<=> record->json [<=> scm->record]] (field getter key ...) ...) : Define a new mapping between a JSON object and a record type, à la SRFI-9.

  • rtd : the name of the record type.

  • ctor : the name for the record constructor procedure.

  • pred : a predicate procedure to check if a given argument holds a record of this type.

  • json->record : the name of the procedure to convert a JSON object, read from a port, string, or alist, into a record of this type.

  • <=> record->json : optional name of the procedure to convert a record of this type to a JSON string.

  • <=> scm->record : optional name of the procedure to convert a JSON object, represented as an alist, into a record of this type. This is equivalent to json->record when an alist is passed.

  • <=> record->scm : optional name of the procedure to convert a record of this type to a JSON objected represented as an alist.

  • ((field getter ...) ...) : a series of field specifications.

    • field : the name of a JSON object field.

    • getter : the name of the procedure to get the value of this field given a record of this type.

    • key : a different name for the field of this JSON object. If given, this name will be used instead of the field name when serializing or deserializing.

    • scm->value : an optional procedure that will be used to convert native values supported by guile-json to the value contained in the record. Used when reading JSON.

    • value->scm : an optional procedure that will be used to convert the value contained in the record to a native value supported by guile-json Used when writing JSON.

Records and null fields

When serializing a record to JSON it is possible to set a field to the *unspecified* value in order to omit it from serialization.

Examples

• Build the string "hello world":

> (scm->json "hello world")
"hello world"

• Build the [1, 2, 3] array:

> (scm->json #(1 2 3))
[1,2,3]

• Build the object { "project" : "foo", "author" : "bar" } using an alist:

> (scm->json '(("project" . "foo") ("author" . "bar")))
{"project":"foo","author":"bar"}

• Build the same object but this time using symbols:

> (scm->json '((project . foo) ("author" . "bar")))
{"project":"foo","author":"bar"}

• Build the object { "values" : [ 234, 98.56 ] }:

> (scm->json '(("values" . #(234 98.56))))
{"values":[234,98.56]}

• Build the object { "values" : [ 234, 98.56 ] } again, this time using a variable:

> (define values #(234 98.56))
> (scm->json `(("values" . ,values)))
{"values":[234,98.56]}

• Default null value is the 'null symbol:

> (scm->json 'null)
null

• The default null value can be changed to something else:

> (scm->json #nil #:null #nil)
null

License

Copyright (C) 2013-2021 Aleix Conchillo Flaque [email protected]

guile-json is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 3 of the License, or (at your option) any later version.

guile-json is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with guile-json. If not, see https://www.gnu.org/licenses/.