Http Monad

A class of Erlang monads which can do http requests with few interesting properties such as composition and laziness.

Inspiration

Microservices have become a design style to evolve system architecture in parallel, implement stable and consistent interfaces. An expressive language is required to design the variety of network communication use-cases. A pure functional languages fits very well to express communication behavior. It gives a rich techniques to hide the networking complexity using monads as abstraction. The IO-monads helps us to compose a chain of network operations and represent them as pure computation, build a new things from small reusable elements. This library uses the "do"-notation, so called monadic binding form. It is well know in functional programming languages such as Haskell, Scala and Erlang. The RESTfull networking becomes a collection of composed "do"-notation in context of a state monad.

The library adapts a human-friendly syntax of HTTP request/response logging/definition used by curl and KATT.

> GET / HTTP/1.1
> Host: example.com
> User-Agent: curl/7.54.0
> Accept: */*
>
< HTTP/1.1 200 OK
< Content-Type: text/html; charset=UTF-8
< Server: ECS (phd/FD58)
< ...

This semantic provides an intuitive approach to specify HTTP requests/responses. Adoption of this syntax as Erlang native code is backed by transformers and provides a rich capability to network programming.

Key features

The interface overview provides an introduction to HTTP monad features and use-cases:

• monad abstraction of HTTP request/response pattern
• supports on-demand, lazy evaluation of HTTP I/O
• high-order composition of individual HTTP requests to complex networking computations
• human-friendly, Erlang native and declarative syntax to depict HTTP operations
• implements a declarative approach for testing of RESTful interfaces
• automatically encodes/decodes Erlang Native HTTP payload using Content-Type hints
• supports generic transformation to algebraic data types (Erlang records)
• supports non-I/O simulation for unit testing

Supported MIME types

• application/json
• application/x-ndjson
• application/x-www-form-urlencoded
• text/plain

Getting started

The latest version of the library is available at its master branch. All development, including new features and bug fixes, take place on the master branch using forking and pull requests as described in contribution guidelines.

Installation

If you are using rebar3 you can include the library in your project with

{deps, [
   {m_http, ".*",
      {git, "https://github.com/fogfish/m_http", {branch, master}}
   }
]}.

Usage

Please study the syntax and HTTP monad interface. The library supplies examples to demonstrate its abilities. This section is a short references to these features. Note, example project uses httpbin.org service.

The following example demonstrates a typical usage scenario. The code uses _ > prefix to declare request (what is going to be send to host) and _ < prefix to declare expected response. The evaluation of "program" fails if expectations do not match actual response.

example() ->
   [m_http ||
      %% request specification
      _ > "GET http://example.com",
      _ > "Accept: text/html",

      %% response specification
      _ < 200,
      _ < "Content-Type: text/html; charset=UTF-8",
      _ < &#39;*&#39;
   ].

Build and run examples with

cd examples
make && make run

Initiate example application

application:ensure_all_started(examples).

Monadic abstraction

Check m_http_syntax.erl, the example guides you thought the basic syntax.

%%
%% builds a pure HTTP networking computation without side-effect evaluation
%%
%% #Fun<m_state.1.102573846>
IO = m_http_syntax:example().

%%
%% evaluate a side-effect of the computation
%%
%% {ok, [#{<<"title">> => <<"Wake up to WonderWidgets!">>, ...}]}
m_http:once(IO).

Composition of HTTP monads

Check m_http_compose.erl, the examples shows the composition practices of multiple requests.

%%
%% builds a pure HTTP networking computation without side-effect evaluation
%%
%% #Fun<m_state.1.102573846>
IO = m_http_compose:hof().

%%
%% evaluate a side-effect of the computation
%%
%% {ok, [#{ua => ..., text => ...}]}
m_http:once(IO).

Encode/Decode

Check m_http_auto_codec.erl, the example illustrates libraries ability to decode content to Erlang native representations, including records.

%%
%% builds a pure HTTP networking computation without side-effect evaluation
%%
%% #Fun<m_state.1.102573846>
IO = m_http_auto_codec:gen().

%%
%% evaluate a side-effect of the computation
%%
%% {ok,#{<<"hello">> => <<"world">>}}
m_http:once(IO).

Previous examples applies automatic codec for generic data structures such as maps, list of pairs. The library also supports semi-auto encode/decode to/from algebraic data structures (records). Please note that current release supports only flats ADTs.

%%
%% builds a pure HTTP networking computation without side-effect evaluation
%%
%% #Fun<m_state.1.102573846>
IO = m_http_auto_codec:adt().

%%
%% evaluate a side-effect of the computation
%%
%% {ok, {adt,<<"world">>}]}
m_http:once(IO).

Declarative testing of RESTfull API

Check httpbin_SUITE.erl. The examples shows a libraries ability to develop a test cases for your RESTfull APIs.

Non I/O simulation

Check m_http_mock_SUITE.erl. The examples demonstrates how to mock HTTP I/O during unit testing.

More Information

• Study category pattern as composition style of development to build a new things from small reusable elements.
• Study syntax and HTTP monad interface
• Study the generic representation.

How To Contribute

The library is Apache 2.0 licensed and accepts contributions via GitHub pull requests:

1. Fork it
2. Create your feature branch (git checkout -b my-new-feature)
3. Commit your changes (git commit -am 'Added some feature')
4. Push to the branch (git push origin my-new-feature)
5. Create new Pull Request

The build process requires Erlang/OTP version 19.0 or later and essential build tools.

Build and run service in your development console. The following command boots Erlang virtual machine and opens Erlang shell.

git clone https://github.com/fogfish/m_http
cd m_http
make
make run

commit message

The commit message helps us to write a good release note, speed-up review process. The message should address two question what changed and why. The project follows the template defined by chapter Contributing to a Project of Git book.

Short (50 chars or less) summary of changes

More detailed explanatory text, if necessary. Wrap it to about 72 characters or so. In some contexts, the first line is treated as the subject of an email and the rest of the text as the body. The blank line separating the summary from the body is critical (unless you omit the body entirely); tools like rebase can get confused if you run the two together.

Further paragraphs come after blank lines.

Bullet points are okay, too

Typically a hyphen or asterisk is used for the bullet, preceded by a single space, with blank lines in between, but conventions vary here

bugs

If you experience any issues with the library, please let us know via GitHub issues. We appreciate detailed and accurate reports that help us to identity and replicate the issue.

• Specify the configuration of your environment. Include which operating system you use and the versions of runtime environments.

• Attach logs, screenshots and exceptions, in possible.

• Reveal the steps you took to reproduce the problem, include code snippet or links to your project.

License