# API for monads
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by Konrad Hinsen

### clojure-contrib is now deprecated

clojure-contrib is no longer being developed or maintained.

Rather than a single, monolithic, contributions library, Clojure now has
a set of separate libraries for each unit of functionality. The libraries
are in the Clojure GitHub organization at
https://github.com/clojure.
API documentation of the libraries can be found at
https://clojure.github.io.

If you're looking for a specific function or namespace from the old
clojure-contrib, see
"Where Did Clojure.Contrib Go".

Full namespace name:

clojure.contrib.monads
## Overview

This library contains the most commonly used monads as well
as macros for defining and using monads and useful monadic
functions.

See also:
Monad tutorial part 1
Monad tutorial part 2
Monad tutorial part 3
Monad tutorial part 4
Monads in Clojure part 1
Monads in Clojure part 2

## Public Variables and Functions

## call-cc

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (call-cc f)

A computation in the cont monad that calls function f with a single
argument representing the current continuation. The function f should
return a continuation (which becomes the return value of call-cc),
or call the passed-in current continuation to terminate.

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## cont-m

var
This library, clojure-contrib, is deprecated. See here for more information.
Monad describing computations in continuation-passing style. The monadic
values are functions that are called with a single argument representing
the continuation of the computation, to which they pass their result.

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## defmonad

macro
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (defmonad name doc-string operations)
(defmonad name operations)

Define a named monad by defining the monad operations. The definitions
are written like bindings to the monad operations m-bind and
m-result (required) and m-zero and m-plus (optional).

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## defmonadfn

macro
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (defmonadfn name docstring? attr-map? args expr)
(defmonadfn name docstring? attr-map? (args expr) ...)

Like defn, but for functions that use monad operations and are used inside
a with-monad block.

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## domonad

macro
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (domonad steps expr)
(domonad name steps expr)

Monad comprehension. Takes the name of a monad, a vector of steps
given as binding-form/monadic-expression pairs, and a result value
specified by expr. The monadic-expression terms can use the binding
variables of the previous steps.
If the monad contains a definition of m-zero, the step list can also
contain conditions of the form :when p, where the predicate p can
contain the binding variables from all previous steps.
A clause of the form :let [binding-form expr ...], where the bindings
are given as a vector as for the use in let, establishes additional
bindings that can be used in the following steps.

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## fetch-state

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (fetch-state)

Return a state-monad function that returns the current state and does not
modify it.

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## fetch-val

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (fetch-val key)

Return a state-monad function that assumes the state to be a map and
returns the value corresponding to the given key. The state is not modified.

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## identity-m

var
This library, clojure-contrib, is deprecated. See here for more information.
Monad describing plain computations. This monad does in fact nothing
at all. It is useful for testing, for combination with monad
transformers, and for code that is parameterized with a monad.

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## m-chain

var
This library, clojure-contrib, is deprecated. See here for more information.
Chains together monadic computation steps that are each functions
of one parameter. Each step is called with the result of the previous
step as its argument. (m-chain (step1 step2)) is equivalent to
(fn [x] (domonad [r1 (step1 x) r2 (step2 r1)] r2)).

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## m-fmap

var
This library, clojure-contrib, is deprecated. See here for more information.
Bind the monadic value m to the function returning (f x) for argument x

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## m-join

var
This library, clojure-contrib, is deprecated. See here for more information.
Converts a monadic value containing a monadic value into a 'simple'
monadic value.

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## m-lift

macro
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (m-lift n f)

Converts a function f of n arguments into a function of n
monadic arguments returning a monadic value.

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## m-map

var
This library, clojure-contrib, is deprecated. See here for more information.
'Executes' the sequence of monadic values resulting from mapping
f onto the values xs. f must return a monadic value.

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## m-reduce

var
This library, clojure-contrib, is deprecated. See here for more information.
Return the reduction of (m-lift 2 f) over the list of monadic values mvs
with initial value (m-result val).

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## m-seq

var
This library, clojure-contrib, is deprecated. See here for more information.
'Executes' the monadic values in ms and returns a sequence of the
basic values contained in them.

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## m-until

var
This library, clojure-contrib, is deprecated. See here for more information.
While (p x) is false, replace x by the value returned by the
monadic computation (f x). Return (m-result x) for the first
x for which (p x) is true.

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## m-when

macro
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (m-when test m-expr)

If test is logical true, return monadic value m-expr, else return
(m-result nil).

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## m-when-not

macro
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (m-when-not test m-expr)

If test if logical false, return monadic value m-expr, else return
(m-result nil).

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## maybe-m

var
This library, clojure-contrib, is deprecated. See here for more information.
Monad describing computations with possible failures. Failure is
represented by nil, any other value is considered valid. As soon as
a step returns nil, the whole computation will yield nil as well.

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## maybe-t

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (maybe-t m)
(maybe-t m nothing)
(maybe-t m nothing which-m-plus)

Monad transformer that transforms a monad m into a monad in which
the base values can be invalid (represented by nothing, which defaults
to nil). The third argument chooses if m-zero and m-plus are inherited
from the base monad (use :m-plus-from-base) or adopt maybe-like
behaviour (use :m-plus-from-transformer). The default is :m-plus-from-base
if the base monad m has a definition for m-plus, and
:m-plus-from-transformer otherwise.

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## monad

macro
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (monad operations)

Define a monad by defining the monad operations. The definitions
are written like bindings to the monad operations m-bind and
m-result (required) and m-zero and m-plus (optional).

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macro
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (monad-transformer base which-m-plus operations)

Define a monad transforer in terms of the monad operations and the base
monad. The argument which-m-plus chooses if m-zero and m-plus are taken
from the base monad or from the transformer.

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## run-cont

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (run-cont c)

Execute the computation c in the cont monad and return its result.

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## sequence-m

var
This library, clojure-contrib, is deprecated. See here for more information.
Monad describing multi-valued computations, i.e. computations
that can yield multiple values. Any object implementing the seq
protocol can be used as a monadic value.

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## sequence-t

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (sequence-t m)
(sequence-t m which-m-plus)

Monad transformer that transforms a monad m into a monad in which
the base values are sequences. The argument which-m-plus chooses
if m-zero and m-plus are inherited from the base monad
(use :m-plus-from-base) or adopt sequence-like
behaviour (use :m-plus-from-transformer). The default is :m-plus-from-base
if the base monad m has a definition for m-plus, and
:m-plus-from-transformer otherwise.

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## set-m

var
This library, clojure-contrib, is deprecated. See here for more information.
Monad describing multi-valued computations, like sequence-m,
but returning sets of results instead of sequences of results.

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## set-state

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (set-state s)

Return a state-monad function that replaces the current state by s and
returns the previous state.

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## set-val

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (set-val key val)

Return a state-monad function that assumes the state to be a map and
replaces the value associated with key by val. The old value is returned.

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## state-m

var
This library, clojure-contrib, is deprecated. See here for more information.
Monad describing stateful computations. The monadic values have the
structure (fn [old-state] [result new-state]).

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## state-m-until

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (state-m-until p f x)

An optimized implementation of m-until for the state monad that
replaces recursion by a loop.

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## state-t

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (state-t m)

Monad transformer that transforms a monad m into a monad of stateful
computations that have the base monad type as their result.

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## update-state

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (update-state f)

Return a state-monad function that replaces the current state by the
result of f applied to the current state and that returns the old state.

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## update-val

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (update-val key f)

Return a state-monad function that assumes the state to be a map and
replaces the value associated with the given key by the return value
of f applied to the old value. The old value is returned.

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## with-monad

macro
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (with-monad monad & exprs)

Evaluates an expression after replacing the keywords defining the
monad operations by the functions associated with these keywords
in the monad definition given by name.

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## with-state-field

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (with-state-field key statement)

Returns a state-monad function that expects a map as its state and
runs statement (another state-monad function) on the state defined by
the map entry corresponding to key. The map entry is updated with the
new state returned by statement.

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## writer-m

function
This library, clojure-contrib, is deprecated. See here for more information.
Usage: (writer-m empty-accumulator)

Monad describing computations that accumulate data on the side, e.g. for
logging. The monadic values have the structure [value log]. Any of the
accumulators from clojure.contrib.accumulators can be used for storing the
log data. Its empty value is passed as a parameter.

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