## Functional Programming Cheat Sheet

Posted on August 10, 2017What follows is translation from traditional programming constructs to how we might express the same, or similar, program using a Functional Programming approach. It is a loose translation, in no particular programming language syntax, then demonstrated in FP using the Haskell programming language.

In some of the examples, there will be reference to a *function*. This can be directly translated to what is used in languages such as Java and C# as an interface. For example, this Java interface…

…used at a call site with the value `n`

…

…can be thought of as a function called `i2s`

applied to a value `n`

, using Haskell syntax:

##### Direct links

- liftA2 on []
- Option/Maybe monad
- liftA2 on reader
- sequence on Maybe with list
- sequence on reader with list
- join on reader
- kleisli composition on Maybe
- kleisli composition on reader
- kleisli composition on State
- sequence on reader with Maybe
- choice on Maybe
- Do effect on condition
- Do effect on not-condition

### liftA2 on []

Given a function `func`

, and two lists `list1`

and `list2`

, loops first through `list1`

, then `list2`

and apply a function to the element at that position.

```
given(func, list1, list2) {
var r = list.empty
for(int i = 0; i < list1.length; i++) {
for(int j = 0; j < list2.length; j++) {
r += list1[i].func(list2[j])
}
}
return r
}
```

This can be written using `liftA2`

which works for many values, including lists. The first argument to `liftA2`

is the function to apply to the list elements. The subsequent arguments are the two lists.

The `liftA2`

function applies to “one loop within one more loop”. You can correlate the number `2`

with the number of loops. There are other `liftA*`

functions for embedding more inner loops.

This general pattern is called *Applicative Functor* and is described in the paper, Applicative Programming With Effects.

### Option/Maybe monad

The `Maybe`

data type, sometimes called the `Option`

data type, can be thought of as a list with a maximum length of one. It is a type-safe substitution for what is often represented as `null`

in other programming languages.

Given two possibly `null`

values, `x`

and `y`

, then if either value is `null`

, return `null`

, otherwise apply a function (such as `+`

) to the two values, knowing that they are not `null`

:

```
given(x, y) {
if (x == null) {
return null
} else if(y == null) {
return null
} else {
return x + y
}
}
```

This can be written using the `>>=`

function, which is overloaded to work on many values, of which `Maybe`

is one instance:

This particular code can also be written using `liftA2`

:

`liftA2 (+) x y`

However, when the second `null`

check depends on the value of the first one, then using the `>>=`

function is required.

For example, given one value `x`

, and a function `test`

, which takes `x`

as argument, for the second `null`

check:

```
given(x, test) {
if (x == null) {
return null
} else {
val y = test(x)
if(y == null) {
return null
} else {
return x + y
}
}
}
```

This can be written using `>>=`

but not `liftA2`

.

### liftA2 on reader

Given two functions `p`

and `q`

, apply them to the same value `x`

, then apply those two results to another function such as `+`

.

Since `liftA2`

is overloaded to work on different structures as long as they all satisfy a similar pattern. We have already seen that lists and `Maybe`

satisfy this pattern. However, functions that accept a single argument also satisfy this pattern. A function that accepts a single argument can also be used to simulate a function that accepts two arguments, by first accepting that one argument, and returning a function that then accepts one argument and takes it to a value. Since in this case we are describing (the simulation of) a * two-argument function* (called

`+`

), then we specifically want the `2`

in `liftA2`

. In common nomenclature, this specific instance is often called the *reader*instance.

### sequence on Maybe with list

Given a list of possibly `null`

values, return either a list of definitely-not `null`

values, or `null`

itself.

```
given(list) {
var r = list.empty
for(int i = 0; i < list.length; i++) {
if(list[i] == null)
return null
else
r.add(list[i])
}
return r
}
```

This can be written using the `sequence`

function, which is overloaded on many values, including `Maybe`

. Since `Maybe`

values have either 0 values or 1 value, it can be used as a type-safe `null`

value. Therefore the `sequence`

function will accept a list of `Maybe`

values, and return a `Maybe`

list of values.

### sequence on reader with list

Given a list of functions (or *instances of an interface*), and a value (`x`

), collect the results of having applied that each function to that value.

```
given(list, x) {
var r = list.empty
for(int i = 0; i < list.length; i++) {
r.add(list[i].apply(x))
}
return r
}
```

Since the `sequence`

function is overloaded, as we have seen it work on `Maybe`

values, it can also work on values that are functions accepting one argument, since that meets the same pattern as `Maybe`

as far as `sequence`

is concerned.

### join on reader

Given a function that accepts two (or more) arguments such as `+`

, apply that function to the same argument.

The `join`

function is overloaded to work on many values, including functions that take one argument. With one call to `join`

, that argument will be passed to a function, then when another function is returned, that same value is passed in again, to return a value. To continue passing this argument, the `join`

function can be *composed* with itself as many times as necessary.

### kleisli composition on Maybe

Given a value (`x`

), one function that might return `null`

(`func1`

), another function that might return `null`

(`func2`

), and a third function that might return `null`

(`func3`

), pass `x`

to `func3`

and only if it does not return `null`

, pass that return result into `func2`

and if that is not `null`

, pass it to `func1`

. If any of the functions return `null`

then return `null`

.

```
given(x) {
func1result = func1(x)
if(func1result == null) {
return null
} else {
func2result = func2(func1result)
if(func2result == null) {
return null
} else {
func3(func2result)
}
}
}
```

The kleisli composition operator, called `<=<`

is overloaded to work on many values, of which `Maybe`

is one instance.

### kleisli composition on reader

Given a value (`x`

), a function (`func1`

) that requires access to a variable (`s`

) to compute its result, another function (`func2`

) that also requires access to `s`

, pass `x`

to `func2`

and the return result of that to `func1`

.

```
global const var s // accessible by func1 and func2
given(x, func1, func2) {
return func1(func2(x))
}
```

We can use the *reader* with kleisli composition.

### kleisli composition on State

The `State`

data structure is structurally equal to a function that passes a value in, potentially modifies it to a new value, and returns another, orthogonal value in association with that state changing. It can be used to to simulate updating variables, while maintaining the principles of Functional Programming.

Given a value (`x`

), a function (`func1`

) that requires access to, and may modify, a variable (`s`

) to compute its result, another function (`func2`

) that also requires access/modify to `s`

, pass `x`

to `func2`

and the return result of that to `func1`

.

```
global var s // accessible/writable by func1 and func2
given(x, func1, func2) {
return func1(func2(x))
}
```

We can use the `State`

data structure to simulate the variable `s`

.

### sequence on reader with Maybe

Given a function (or *instance of an interface*), called `func`

, which may be `null`

, and a value (`x`

), apply the function to the value. If the function is `null`

, return `null`

.

The possibility for `null`

can be simulated using the `Maybe`

data type. Since `Maybe`

can be treated as a list with a maximum length of one, we can also *sequence* this, and we can do it with many values that meet a certain pattern. A function that accepts one argument meets this pattern, so we can use `sequence`

for this case.

### choice on Maybe

Given two values (`x`

and `y`

), then if `x`

is not `null`

, return it, otherwise try `y`

.

In some programming languages, this expression might be written using a ternary operator:

The C# programming language has a built-in operator (`??`

) for this, called the elvis operator.

The choice operator in Haskell is called `<|>`

and is overloaded to work on many different values, of which `Maybe`

is one instance.

### Do effect on condition

Given some boolean condition, if it is true, do something. If it is false, do nothing.

Since Haskell values are always pure, including effects, they can be treated as values. That is, they can be passed to functions as arguments.

### Do effect on not-condition

Given some boolean condition, if it is false, do something. If it is true, do nothing.

The `when`

and `unless`

functions can handled performing some effect based on a condition. Since the condition is negated, we want the `unless`

function.

Can you come up with any more patterns that translate in a helpful way? Let us know if you do.

#### > Tony Morris

Tony Morris is a programmer at the Queensland Functional Programming Lab.