# Recursion

The use of self-reference in defining abstractions

## Illustration

Clearly, there are different forms of recursions, as they are different abstraction mechanisms that permit recursion. For instance, in functional programming, both Functions and data types may be defined recursively.

### Recursive functions

Consider the following recursive formulation of the factorial function in Language:Haskell:

```-- A recursive definition of the factorial function
factorial n =
if n==0
then 1
else n * factorial (n-1)
```

This is essentially a form of primitive recursion: the function definition checks for the argument n to be "0" for the base case and applies the function recursively to the predecessor of the argument otherwise. For the record, non-recursive formulations are feasible, too, depending on the helper functions we are willing to use. For instance, we may use the ".." operator to enumerate all values in a range and then apply the product function:

```-- A non-recursive definition of the factorial function
factorial' n = product [1..n]
```

this

Marcel Heinz edited this article at Fri, 01 Jun 2018 13:53:59 +0200

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