Functional Programming Patterns

TIL

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Functional Programming

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checkraiser viết ngày 27/06/2018

Functional Programming Patterns

• Adding a function parameter (Parameterization)
• Passing a function parameter (Currying)
• Applying a function ( $ )
• Composing a function: applies a succession of transformations to supplied data ((.))
• Folding: reduces a sequence of values to a single value by combining adjacent pairs (foldr1)
• Mapping: applies the same transformation to each value in a sequence (fmap)
• Iteration: applies the same transformation repeatedly to its own delivered values, building a sequence of successively more refined iterates (iterate)

iterate f x = [d0, d1, d2,..]
    where [d0, d1, d2,...] = [x, f x, (f . f) x, ...]

• Zipping: delivers a sequence whose length matches that of the shorter of the sequences supplied as its last two arguments. Zipping stops when one of those arguments runs out of elements.

zipWith op [x1, x2, x3, ..] [y1, y2 , y3 , ...] =
    [op x1 y1, op x2 y2 , op x3 y3 , ... ]

Example:

fibs :: [Int]
fibs = 0 : 1 : zipWith (+) fibs (tail fibs)

• Filtering: selecting certain elements from a given sequence to form a new one.

-- Filter as Folding:

filter keep = foldr op []
    where 
    op x ys 
        | keep x = [x] ++ ys
        | otherwise = ys 

Note: When you are trying to describe a computation that involves repetition, try to view it as one of common patterns: mapping, folding, filtering, iteration, and zipping. The operators map, foldr, filter, iterate, and zipWith make up a kind of linguistic shorthand that covers probably over 90% of the computations involving repetition that you will encounter in practice.

• Polymorphic: operate on many different types of arguments (reverse)
• Type Class: collection of types that share a collection of functions and/or operations (Eq), for example, with Eq class, it made comparision possible.
• Higher order function: Functions that have an argument that is, itself, a function are called higher-order functions. Functions that deliver a value that is, itself, a function are also called higher-order functions. The composition operator is a higher-order function that does both.
• Encapsulation: Isolating components of software so that modifying internal details in one com- ponent will have no affect on other components of the software (where)

checkraiser 23-06-2018