Functional Programming

Functional Programming

• Code that is declarative
  • The program is a series of declarations
  • There is no changing of state or mutable data
• Functional programming is a subset of declarative programming
• It sees the world as a collection of mathematical functions
  • The result of a function depends only on its arguments

What is a function?

• A function is a mathematical expression that will always give the same output when presented with the same inputs.

Why functional programming?

• It is more robust
• It is easier to reason about
• Less prone to errors
• Has no side effects, even as the code base increases

Features of functional programming

• Concurrency
• Lazy evaluation
• Recursion
• Data is immutable

Partial Evaluation

• This is an optimisation method used to improve the performance of a program

• Consider a function with two inputs

  • find name scores
    • This is intended to find the scores from the list scores for the person named by the variable name in a database of names
    • The link between them might be candidate_id

• With a partial evaluation this can be converted into two function calls, the total processing effort for the two calls is less than the time to process the single, more complicated call.

  • So, in our example we might break this down into two stages - the first where we find the name in the database, and the second where we find the scores.
  • The performance boost comes because we might be able to pre-process the first stage - partial evaluation of the whole expression.

• This works best if one of the inputs can be fixed.

  • At run time we might know one of the values and it will not change during this execution.
    • Think about a date, for example.
  • So, before we get the command, we can do the required processing that the “date” part of the operation depends on.

• Consider the command add 2 age

• We know that the function always adds 2.

• So lets create a temporary function called add2.

• When we get to this command in the code we don’t run it, we run:

  • add2 age

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