This post is the third in a series.
In the previous two posts, I described some of the core functions for dealing with generic data types: `map`

, `apply`

, `bind`

, and so on.

In this post, I’ll show how to use these functions in practice, and will explain the difference between the so-called “applicative” and “monadic” styles.

Here’s a list of shortcuts to the various functions mentioned in this series:

**Part 1: Lifting to the elevated world****Part 2: How to compose world-crossing functions****Part 3: Using the core functions in practice****Part 4: Mixing lists and elevated values****Part 5: A real-world example that uses all the techniques****Part 6: Designing your own elevated world****Part 7: Summary**

Now that we have the basic tools for lifting normal values to elevated values and working with cross-world functions, it’s time to start working with them!

In this section, we’ll look at some examples how these functions are actually used.

I briefly mentioned earlier that there is a important difference between using `apply`

and `bind`

. Let’s go into this now.

When using `apply`

, you can see that each parameter (`E<a>`

, `E<b>`

) is completely independent of the other. The value of `E<b>`

does not depend on what `E<a>`

is.

On the other hand, when using `bind`

, the value of `E<b>`

*does* depend on what `E<a>`

is.

The distinction between working with independent values or dependent values leads to two different styles:

- The so-called “applicative” style uses functions such as
`apply`

,`lift`

, and`combine`

where each elevated value is independent. - The so-called “monadic” style uses functions such as
`bind`

to chain together functions that are dependent on a previous value.

What does that mean in practice? Well, let’s look at an example where you could choose from both approaches.

Say that you have to download data from three websites and combine them. And say that we have an action, say `GetURL`

, that gets the data from a website on demand.

Now you have a choice:

**Do you want to fetch all the URLs in parallel?**If so, treat the`GetURL`

s as independent data and use the applicative style.**Do you want to fetch each URL one at a time, and skip the next in line if the previous one fails?**If so, treat the`GetURL`

s as dependent data and use the monadic style. This linear approach will be slower overall than the “applicative” version above, but will also avoid unnecessary I/O.**Does the URL for the next site depend on what you download from the previous site?**In this case, you are*forced*to use “monadic” style, because each`GetURL`

depends on the output of the previous one.

As you can see, the choice between applicative style and monadic style is not clear cut; it depends on what you want to do.

We’ll look at a real implementation of this example in the final post of this series.

**but…**

It’s important to say that just because you *choose* a style doesn’t mean it will be implemented as you expect.
As we have seen, you can easily implement `apply`

in terms of `bind`

, so even if you use `<*>`

in your code, the implementation may be proceeding monadically.

In the example above, the implementation does not have to run the downloads in parallel. It could run them serially instead.
By using applicative style, you’re just saying that you don’t care about dependencies and so they *could* be downloaded in parallel.

If you use the applicative style, that means that you define all the actions up front – “statically”, as it were.

In the downloading example, the applicative style requires that you specific *in advance* which URLs will be visited.
And because there is more knowledge up front it means that we can potentially do things like parallelization or other optimizations.

On the other hand, the monadic style means that only the initial action is known up front. The remainder of the actions are determined dynamically, based on the output of previous actions. This is more flexible, but also limits our ability to see the big picture in advance.

Sometimes *dependency* is confused with *order of evaluation*.

Certainly, if one value depends on another then the first value must be evaluated before the second value. And in theory, if the values are completely independent (and have no side effects), then they can be evaluated in any order.

However, even if the values are completely independent, there can still be an *implicit* order in how they are evaluated.

For example, even if the list of `GetURL`

s is done in parallel,
it’s likely that the urls will begin to be fetched in the order in which they are listed, starting with the first one.

And in the `List.apply`

implemented in the previous post, we saw that `[f; g] apply [x; y]`

resulted in `[f x; f y; g x; g y]`

rather than `[f x; g x; f y; g y]`

.
That is, all the `f`

values are first, then all the `g`

values.

In general, then, there is a convention that values are evaluated in a left to right order, even if they are independent.

To see how both the applicative style and monadic style can be used, let’s look at an example using validation.

Say that we have a simple domain containing a `CustomerId`

, an `EmailAddress`

, and a `CustomerInfo`

which is a record containing both of these.

```
type CustomerId = CustomerId of int
type EmailAddress = EmailAddress of string
type CustomerInfo = {
id: CustomerId
email: EmailAddress
}
```

And let’s say that there is some validation around creating a `CustomerId`

. For example, that the inner `int`

must be positive.
And of course, there will be some validation around creating a `EmailAddress`

too. For example, that it must contain an “@” sign at least.

How would we do this?

First we create a type to represent the success/failure of validation.

```
type Result<'a> =
| Success of 'a
| Failure of string list
```

Note that I have defined the `Failure`

case to contain a *list* of strings, not just one. This will become important later.

With `Result`

in hand, we can go ahead and define the two constructor/validation functions as required:

```
let createCustomerId id =
if id > 0 then
Success (CustomerId id)
else
Failure ["CustomerId must be positive"]
// int -> Result<CustomerId>
let createEmailAddress str =
if System.String.IsNullOrEmpty(str) then
Failure ["Email must not be empty"]
elif str.Contains("@") then
Success (EmailAddress str)
else
Failure ["Email must contain @-sign"]
// string -> Result<EmailAddress>
```

Notice that `createCustomerId`

has type `int -> Result<CustomerId>`

, and `createEmailAddress`

has type `string -> Result<EmailAddress>`

.

That means that both of these validation functions are world-crossing functions, going from the normal world to the `Result<_>`

world.

`Result`

Since we are dealing with world-crossing functions, we know that we will have to use functions like `apply`

and `bind`

, so let’s define them for our `Result`

type.

```
module Result =
let map f xResult =
match xResult with
| Success x ->
Success (f x)
| Failure errs ->
Failure errs
// Signature: ('a -> 'b) -> Result<'a> -> Result<'b>
// "return" is a keyword in F#, so abbreviate it
let retn x =
Success x
// Signature: 'a -> Result<'a>
let apply fResult xResult =
match fResult,xResult with
| Success f, Success x ->
Success (f x)
| Failure errs, Success x ->
Failure errs
| Success f, Failure errs ->
Failure errs
| Failure errs1, Failure errs2 ->
// concat both lists of errors
Failure (List.concat [errs1; errs2])
// Signature: Result<('a -> 'b)> -> Result<'a> -> Result<'b>
let bind f xResult =
match xResult with
| Success x ->
f x
| Failure errs ->
Failure errs
// Signature: ('a -> Result<'b>) -> Result<'a> -> Result<'b>
```

If we check the signatures, we can see that they are exactly as we want:

`map`

has signature:`('a -> 'b) -> Result<'a> -> Result<'b>`

`retn`

has signature:`'a -> Result<'a>`

`apply`

has signature:`Result<('a -> 'b)> -> Result<'a> -> Result<'b>`

`bind`

has signature:`('a -> Result<'b>) -> Result<'a> -> Result<'b>`

I defined a `retn`

function in the module to be consistent, but I don’t bother to use it very often. The *concept* of `return`

is important,
but in practice, I’ll probably just use the `Success`

constructor directly. In languages with type classes, such as Haskell, `return`

is used much more.

Also note that `apply`

will concat the error messages from each side if both parameters are failures.
This allows us to collect all the failures without discarding any. This is the reason why I made the `Failure`

case have a list of strings, rather than a single string.

*NOTE: I’m using string for the failure case to make the demonstration easier. In a more sophisticated design I would list the possible failures explicitly.
See my functional error handling talk for more details.*

Now that we have the domain and the toolset around `Result`

, let’s try using the applicative style to create a `CustomerInfo`

record.

The outputs of the validation are already elevated to `Result`

, so we know we’ll need to use some sort of “lifting” approach to work with them.

First we’ll create a function in the normal world that creates a `CustomerInfo`

record given a normal `CustomerId`

and a normal `EmailAddress`

:

```
let createCustomer customerId email =
{ id=customerId; email=email }
// CustomerId -> EmailAddress -> CustomerInfo
```

Note that the signature is `CustomerId -> EmailAddress -> CustomerInfo`

.

Now we can use the lifting technique with `<!>`

and `<*>`

that was explained in the previous post:

```
let (<!>) = Result.map
let (<*>) = Result.apply
// applicative version
let createCustomerResultA id email =
let idResult = createCustomerId id
let emailResult = createEmailAddress email
createCustomer <!> idResult <*> emailResult
// int -> string -> Result<CustomerInfo>
```

The signature of this shows that we start with a normal `int`

and `string`

and return a `Result<CustomerInfo>`

Let’s try it out with some good and bad data:

```
let goodId = 1
let badId = 0
let goodEmail = "test@example.com"
let badEmail = "example.com"
let goodCustomerA =
createCustomerResultA goodId goodEmail
// Result<CustomerInfo> =
// Success {id = CustomerId 1; email = EmailAddress "test@example.com";}
let badCustomerA =
createCustomerResultA badId badEmail
// Result<CustomerInfo> =
// Failure ["CustomerId must be positive"; "Email must contain @-sign"]
```

The `goodCustomerA`

is a `Success`

and contains the right data, but the `badCustomerA`

is a `Failure`

and contains two validation error messages. Excellent!

Now let’s do another implementation, but this time using monadic style. In this version the logic will be:

- try to convert an int into a
`CustomerId`

- if that is successful, try to convert a string into a
`EmailAddress`

- if that is successful, create a
`CustomerInfo`

from the customerId and email.

Here’s the code:

```
let (>>=) x f = Result.bind f x
// monadic version
let createCustomerResultM id email =
createCustomerId id >>= (fun customerId ->
createEmailAddress email >>= (fun emailAddress ->
let customer = createCustomer customerId emailAddress
Success customer
))
// int -> string -> Result<CustomerInfo>
```

The signature of the monadic-style `createCustomerResultM`

is exactly the same as the applicative-style `createCustomerResultA`

but internally it is doing something different,
which will be reflected in the different results we get.

```
let goodCustomerM =
createCustomerResultM goodId goodEmail
// Result<CustomerInfo> =
// Success {id = CustomerId 1; email = EmailAddress "test@example.com";}
let badCustomerM =
createCustomerResultM badId badEmail
// Result<CustomerInfo> =
// Failure ["CustomerId must be positive"]
```

In the good customer case, the end result is the same, but in the bad customer case, only *one* error is returned, the first one.
The rest of the validation was short circuited after the `CustomerId`

creation failed.

This example has demonstrated the difference between applicative and monadic style quite well, I think.

- The
*applicative*example did all the validations up front, and then combined the results. The benefit was that we didn’t lose any of the validation errors. The downside was we did work that we might not have needed to do.

- On the other hand, the monadic example did one validation at a time, chained together.
The benefit was that we short-circuited the rest of the chain as soon as an error occurred and avoided extra work.
The downside was that we only got the
*first*error.

Now there is nothing to say that we can’t mix and match applicative and monadic styles.

For example, we might build a `CustomerInfo`

using applicative style, so that we don’t lose any errors,
but later on in the program, when a validation is followed by a database update,
we probably want to use monadic style, so that the database update is skipped if the validation fails.

Finally, let’s build a computation expression for these `Result`

types.

To do this, we just define a class with members called `Return`

and `Bind`

, and then we create an instance of that class, called `result`

, say:

```
module Result =
type ResultBuilder() =
member this.Return x = retn x
member this.Bind(x,f) = bind f x
let result = new ResultBuilder()
```

We can then rewrite the `createCustomerResultM`

function to look like this:

```
let createCustomerResultCE id email = result {
let! customerId = createCustomerId id
let! emailAddress = createEmailAddress email
let customer = createCustomer customerId emailAddress
return customer }
```

This computation expression version looks almost like using an imperative language.

Note that F# computation expressions are always monadic, as is Haskell do-notation and Scala for-comprehensions. That’s not generally a problem, because if you need applicative style it is very easy to write without any language support.

In practice, we often have a mish-mash of different kinds of values and functions that we need to combine together.

The trick for doing this is to convert all them to the *same* type, after which they can be combined easily.

Let’s revisit the previous validation example, but let’s change the record so that it has an extra property, a `name`

of type string:

```
type CustomerId = CustomerId of int
type EmailAddress = EmailAddress of string
type CustomerInfo = {
id: CustomerId
name: string // New!
email: EmailAddress
}
```

As before, we want to create a function in the normal world that we will later lift to the `Result`

world.

```
let createCustomer customerId name email =
{ id=customerId; name=name; email=email }
// CustomerId -> String -> EmailAddress -> CustomerInfo
```

Now we are ready to update the lifted `createCustomer`

with the extra parameter:

```
let (<!>) = Result.map
let (<*>) = Result.apply
let createCustomerResultA id name email =
let idResult = createCustomerId id
let emailResult = createEmailAddress email
createCustomer <!> idResult <*> name <*> emailResult
// ERROR ~~~~
```

But this won’t compile! In the series of parameters `idResult <*> name <*> emailResult`

one of them is not like the others.
The problem is that `idResult`

and `emailResult`

are both Results, but `name`

is still a string.

The fix is just to lift `name`

into the world of results (say `nameResult`

) by using `return`

, which for `Result`

is just `Success`

.
Here is the corrected version of the function that does work:

```
let createCustomerResultA id name email =
let idResult = createCustomerId id
let emailResult = createEmailAddress email
let nameResult = Success name // lift name to Result
createCustomer <!> idResult <*> nameResult <*> emailResult
```

The same trick can be used with functions too.

For example, let’s say that we have a simple customer update workflow with four steps:

- First, we validate the input. The output of this is the same kind of
`Result`

type we created above. Note that this validation function could*itself*be the result of combining other, smaller validation functions using`apply`

. - Next, we canonicalize the data. For example: lowercasing emails, trimming whitespace, etc. This step never raises an error.
- Next, we fetch the existing record from the database. For example, getting a customer for the
`CustomerId`

. This step could fail with an error too. - Finally, we update the database. This step is a “dead-end” function – there is no output.

For error handling, I like to think of there being two tracks: a Success track and a Failure track. In this model, an error-generating function is analogous to a railway switch (US) or points (UK).

The problem is that these functions cannot be glued together; they are all different shapes.

The solution is to convert all of them to the *same* shape, in this case the two-track model with success and failure on different tracks.
Let’s call this *Two-Track world*!

Each original function, then, needs to be elevated to Two-Track world, and we know just the tools that can do this!

The `Canonicalize`

function is a single track function. We can turn it into a two-track function using `map`

.

The `DbFetch`

function is a world-crossing function. We can turn it into a wholly two-track function using `bind`

.

The `DbUpdate`

function is more complicated. We don’t like dead-end functions, so first we need to transform it to a function where the data keeps flowing.
I’ll call this function `tee`

. The output of `tee`

has one track in and one track out, so we need to convert it to a two-track function, again using `map`

.

After all these transformations, we can reassemble the new versions of these functions. The result looks like this:

And of course, these functions can now be composed together very easily, so that we end up with a single function looking like this, with one input and a success/failure output:

This combined function is yet another world-crossing function of the form `a->Result<b>`

, and so it in turn can be used as a component part of a even bigger function.

For more examples of this “elevating everything to the same world” approach, see my posts on functional error handling and threading state.

There is an alternative world which can be used as a basic for consistency which I will call “Kleisli” world, named after Professor Kleisli – a mathematician, of course!

In Kleisli world *everything* is a cross-world function! Or, using the railway track analogy, everything is a switch (or points).

In Kleisli world, the cross-world functions *can* be composed directly,
using an operator called `>=>`

for left-to-right composition or `<=<`

for right-to-left composition.

Using the same example as before, we can lift all our functions to Kleisli world.

- The
`Validate`

and`DbFetch`

functions are already in the right form so they don’t need to be changed. - The one-track
`Canonicalize`

function can be lifted to a switch just by lifting the output to a two-track value. Let’s call this`toSwitch`

.

- The tee-d
`DbUpdate`

function can be also lifted to a switch just by doing`toSwitch`

after the tee.

Once all the functions have been lifted to Kleisli world, they can be composed with Kleisli composition:

Kleisli world has some nice properties that Two-Track world doesn’t but on the other hand, I find it hard to get my head around it! So I generally stick to using Two-Track world as my foundation for things like this.

In this post, we learned about “applicative” vs “monadic” style, and why the choice could have an important effect on which actions are executed, and what results are returned.

We also saw how to lift different kinds values and functions to a consistent world so that our functions can be composed more easily.

In the next post we’ll look at a common problem: working with lists of elevated values.