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102 lines
2.6 KiB
ReStructuredText
102 lines
2.6 KiB
ReStructuredText
###########
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calrissian
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###########
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.. image:: https://travis-ci.org/correl/calrissian.svg?branch=master :target: https://travis-ci.org/correl/calrissian
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Introduction
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============
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Calrissian is an implementation of monads in LFE, inspired by
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`erlando`_, mostly as a learning exercise. The following monads are currently supported:
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* Identity
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* Maybe
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* Error
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* State
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* State Transformer
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Dependencies
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------------
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This project assumes that you have `rebar`_ installed somwhere in your
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``$PATH``.
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This project depends upon the following, which are installed to the ``deps``
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directory of this project when you run ``make deps``:
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* `LFE`_ (Lisp Flavored Erlang; needed only to compile)
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* `lfeunit`_ (needed only to run the unit tests)
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Installation
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============
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Just add it to your ``rebar.config`` deps:
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.. code:: erlang
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{deps, [
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...
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{calrissian, ".*", {git, "git@github.com:correl/calrissian.git", "master"}}
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]}.
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And then do the usual:
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.. code:: bash
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$ rebar get-deps
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$ rebar compile
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Examples
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========
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The following examples demonstrate some of the possible uses of monads
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in real-world code.
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Error Monad
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-----------
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The following is an example of using the error monad and do-notation
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to simplify flow control through a series of sequential operations
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that, if any step should fail, should halt execution and return an
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error.
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The error monad will inspect the result of the previous operation. If
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it was successful (represented as ``'ok`` or ``(tuple 'ok result)``),
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the result will be passed on to the next operation. If it failed
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(represented as ``(tuple 'error reason)``, the error will be returned
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and execution will cease.
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.. code:: scheme
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(include-lib "calrissian/include/monads.lfe")
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(defun dostuff ()
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(do-m (monad 'error)
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(input <- (fetch-input)) ;; fetch-input -> (tuple 'ok result) | (tuple 'error reason)
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(parsed <- (parse-input input)) ;; parse-input -> (tuple 'ok result) | (tuple 'error reason)
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(store-data parsed))) ;; store-data -> 'ok | (tuple 'error reason)
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Without the error monad, the code might have looked like this:
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.. code:: scheme
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(defun dostuff ()
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(case (fetch-input)
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((tuple 'error reason)
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(tuple 'error reason))
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((tuple 'ok input)
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(case (parse-input input)
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((tuple 'error reason)
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(tuple 'error reason))
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((tuple 'ok parsed)
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(store-data parsed))))))
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.. Links
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.. -----
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.. _erlando: https://github.com/rabbitmq/erlando
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.. _rebar: https://github.com/rebar/rebar
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.. _LFE: https://github.com/rvirding/lfe
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.. _lfeunit: https://github.com/lfe/lfeunit
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