Functional programming in Ruby

Alex Teutaka

@jaturken

Functional Programming

A programming paradigm that treats computation as the evaluation of functions

and avoids state and mutable data.

Imperative Programming

A programming paradigm that describes computation in terms of statements that

change a program state.

A programming paradigm that treats computation as the evaluation of functions

and avoids state and mutable data.

The difference

Imperative functions can have side effects that may change the value of program

state, so this approach looses...

A programming paradigm that treats computation as the evaluation of functions

and avoids state and mutable data.

A programming paradigm that describes computation in terms of statements that

change a program state.

Referential transparency

The same language expression can result in different values at different times

depending on the state of the executing program.

Pure Functions

● Always evaluates the same result value given the same argument values

● Evaluation of the result does not cause any side effect

Pure

● sin● to_s● Enumerable#select● Enumerable#collect● Array#uniq● Hash#merge

Non-pure

Side-effects:● print● require● Enumerable#select!● Enumerable#collect!● Array#uniq!● Hash#merge!● Hash#delete

Non-determinated:● Date.today● rand● Externall API

calls

Pure Functions is Good

● Can be cached/memoized● Order of calculation does not matter● Any evaluation strategy can be used● Chain can be easily paralellized

Functional Programming

X = X + 1

Functional Programming

X = X + 1

Iterations without iterator?

Recursion

Example

Tail Call

A special case of recursion when recursive call of function itself is its last operation.

Example

Expand example code

tail_call_factorial:

(fact-tail 3 1)

(fact-tail 2 3)

(fact-tail 1 6)

6

non_tail_call_factorial:

(fact 3)

(* 3 (fact 2))

(* 3 (* 2 (fact 1)))

(* 3 (* 2 1))

(* 3 2)

6

Tail Call Optimization

Tail call is equivalent to iteration. So Tail Call Optimization(TCO) is evaluating of

Tail Call as an iteration. In Ruby TCO is turned off by default.

First Class Objects

● can be stored in values and data structures

● can be passed as a parameter to an expression

● can be returned as the result of a expression

Higher-order Functions

Functions that can either take other functions as arguments or return them as

results

Closure

A function or reference to a function together with a referencing environment

Example

Currying

The technique of transforming a function that takes multiple arguments to a chain of

functions each with a single argument.

Without currying

With currying

Lazy Evaluation

An evaluation strategy which delays the evaluation of an expression until its value

is needed

Example

Advantages of Functional Programming

● Reliability● Parallelizm● Easy unit testing● Easy debugging

Disadvantages of Functional Programming

● Seems to be more difficult● Typically less efficient● Garbage collector needed● Difficulties with IO methods and states

Thank you for attention!