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morph

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A Library of Morphisms: Monoids, Functors, and Monads

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# Morph

Morph is a library of Haskell-style morphisms: monoids, functors, and monads.
These constructs are helpful for designing programs that are purely functional
and that encapsulate the boilerplate employed by many programming techniques.

## Features

* Implementation based on protocols and data types.
* Predefined monoids and functors; with support for Clojure collections.
* Monads: Identity, Maybe, Either, Reader, Writer, State, Imperative.
* Monad Transformers: MaybeT, EitherT, ReaderT, WriterT, StateT.
* Support for curried functions.
* Library of generic functions for the above constructs.
* Sample code in `src/main/resources`.

These constructs have a reputation of being hard to explain and even harder to 
understand and to apply in everyday programming. I've made every effort to
present them as regular techniques and idioms with practical benefits. Behind
their strange or fancy names, these are just functions that work on data types.

An intuition of their behavior is all that's needed to take advantage of these
functions; you may never need or want to write your own. I'm pleased with the
power and simplicity these techniques have to offer and I hope you may find
them useful as well.

## Sample Usage

A simple expression evaluator uses the State monad; it can declare variables and clear the symbol table. Access to the symbol table is direct as if it were global, though all functions are pure.
```clojure
(use '[blancas.morph core monads])

(def table {'DEG 57.295779 'E 2.718281 'PI 3.141592})

(declare run)

(defn calc
  "Evaluates and unboxes the operands; then applies the
   operator and returns the result as new state value."
  [op x y]
  (monad [a (run x) b (run y)]
    (state (op a b))))

(defn const
  "Returns a state value with a looked-up value or the given number."
  [x] (if (symbol? x) (gets x) (state x)))

(defn decl
  "Declares a variable as (var = val); value is used in-place."
  [x y] (>> (modify-state assoc x y) (state y)))

(defn clear
  "Clears the symbol table as (val %); value is used in-place."
  [x] (>> (put-state {}) (state x)))

(defn run
  "Dispatches on an expression or a single value."
  [op]
  (if (list? op)
    (case (second op)
      + (calc + (first op) (last op))
      - (calc - (first op) (last op))
      * (calc * (first op) (last op))
      / (calc / (first op) (last op))
      = (decl   (first op) (last op))
      % (clear  (first op)))
    (const op)))
```

Now we evaluate some expressions with `table` as the initial state.
```clojure
(eval-state (run '((9 / 3) + (2 * (PI - E)))) table)
;; 3.846622
(exec-state (run '((180 / (k = 30)) + (k * (PI - E)))) table)
;; {PI 3.141592, E 2.718281, DEG 57.295779, k 30}
(exec-state (run '(((180 %) / (k = 30)) + ((j = 5) * (k - j)))) table)
;; {j 5, k 30}
```

## Setup

Leiningen:

```clojure
[org.blancas/morph "0.3.0"]
```

Maven:

```xml
<dependency>
  <groupId>org.blancas</groupId>
  <artifactId>morph</artifactId>
  <version>0.3.0</version>
</dependency>
```

### Changes for release 0.3.0:

* Fixed slow, reflective calls by adding type hints.

## Documentation

Browse the whole [change log](https://github.com/blancas/morph/wiki/Change-Log).

Morph is documented in the [Wiki](https://github.com/blancas/morph/wiki).

Browse the Codox [Morph v0.3.0 API](http://blancas.github.com/morph).

To generate the API docs (in the `codox` directory):

lein doc

## YourKit

YourKit is kindly supporting open source projects with its full-featured Java
Profiler.

YourKit, LLC is the creator of innovative and intelligent tools for profiling
Java and .NET applications. Take a look at YourKit's leading software products:

* <a href="http://www.yourkit.com/java/profiler/index.jsp">YourKit Java Profiler</a> and
* <a href="http://www.yourkit.com/.net/profiler/index.jsp">YourKit .NET Profiler</a>.

## License

Licensed under the [Eclipse Public License](http://www.eclipse.org/legal/epl-v10.html).

morph

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