Railway Oriented Programming

Railway Oriented Programming
A functional approach to error handling
What do railways
have to do with
programming?

Railway Oriented Programming
A functional approach to error handling
Scott Wlaschin
@ScottWlaschin
fsharpforfunandprofit.c...

Overview
Topics covered:
• Happy path programming
• Straying from the happy path
• Introducing "Railway Oriented Programmi...

Happy path programming
Implementing a simple use case

A simple use case
Receive request
Validate and canonicalize request
Update existing user record
Send verification email
Re...

Imperative code
string ExecuteUseCase()
{
var request = receiveRequest();
validateRequest(request);
canonicalizeEmail(requ...

Functional flow
let executeUseCase =
receiveRequest
>> validateRequest
>> canonicalizeEmail
>> updateDbFromRequest
>> send...

Straying from the happy path...
What do you do when
something goes wrong?

Straying from the happy path

“A program is a spell cast over a
computer, turning input
into error messages”

“A program is a spell cast over a
computer, turning input
into error messages”

Straying from the happy path
Name is blank
Email not valid
Receive request
Validate and canonicalize request
Update existi...

Imperative code with error handling
string UpdateCustomerWithErrorHandling()
{
var request = receiveRequest();
validateReq...

Imperative code with error handling
string UpdateCustomerWithErrorHandling()
{
var request = receiveRequest();
var isValid...

Imperative code with error handling
string UpdateCustomerWithErrorHandling()
{
var request = receiveRequest();
var isValid...

Imperative code with error handling
string UpdateCustomerWithErrorHandling()
{
var request = receiveRequest();
var isValid...

Imperative code with error handling
string UpdateCustomerWithErrorHandling()
{
var request = receiveRequest();
var isValid...

Imperative code with error handling
string UpdateCustomerWithErrorHandling()
{
var request = receiveRequest();
var isValid...

Functional flow with error handling
let updateCustomer =
receiveRequest
>> validateRequest
>> canonicalizeEmail
>> updateD...

Functional flow with error handling
let updateCustomerWithErrorHandling =
receiveRequest
>> validateRequest
>> canonicaliz...

Designing the unhappy path

Request/response (non-functional) design
Request
Response
Validate Update Send
Request handling service
Request
Response
V...

Data flow (functional) design
ResponseValidate Update SendA single function representing the use caseRequest
Request Respo...

Functional design
How can a function have more than one output?
type Result =
| Success
| ValidationError
| UpdateError
| ...

Functional design
How can a function have more than one output?
type Result =
| Success
| Failure
Request
Errors
SuccessVa...

Functional design
How can a function have more than one output?
type Result<'TEntity> =
| Success of 'TEntity
| Failure of...

Functional design
Request
Errors
SuccessValidate Update Send
Failure
A single function representing the use case
• Each us...

How do I work with errors
in a functional way?

Monad dialog

Monads are confusing?
Monads are not really that confusing
http://bit.ly/monad-paper

Railway oriented programming
This has absolutely nothing to do with monads.

A railway track analogy
The Tunnel of
Transformation
Function
apple -> banana

A railway track analogy
Function 1
apple -> banana
Function 2
banana -> cherry

A railway track analogy
>>
Function 1
apple -> banana
Function 2
banana -> cherry

A railway track analogy
New Function
apple -> cherry
Can't tell it was built from
smaller functions!

An error generating function
Request SuccessValidate
Failure
let validateInput input =
if input.name = "" then
Failure "Na...

Introducing switches
Success!
Failure
Input ->

Connecting switches
Validate UpdateDbon success
bypass

Connecting switches
Validate UpdateDb

Connecting switches
Validate UpdateDb SendEmail

Connecting switches
Validate UpdateDb SendEmail

The two-track model in practice

Composing switches
Validate UpdateDb SendEmail

Composing switches
Validate UpdateDb SendEmail

Composing switches
Validate UpdateDb SendEmail
>> >>
Composing one-track functions is fine...

Composing switches
Validate UpdateDb SendEmail
>> >>
... and composing two-track functions is fine...

Composing switches
Validate UpdateDb SendEmail
 
... but composing switches is not allowed!

Composing switches
Validate
Two-track input Two-track input
Validate
One-track input Two-track input



Building an adapter block
Two-track input
Slot for switch function
Two-track output

Building an adapter block
Two-track input Two-track output
Validate Validate

let adapt switchFunction =
fun twoTrackInput ->
match twoTrackInput with
| Success s -> switchFunction s
| Failure f -> Fa...

let adapt switchFunction =
fun twoTrackInput ->
match twoTrackInput with
| Success s -> switchFunction s
| Failure f -> Fa...

let adapt switchFunction =
fun twoTrackInput ->
match twoTrackInput with
| Success s -> switchFunction s
| Failure f -> Fa...

let adapt switchFunction =
fun twoTrackInput ->
match twoTrackInput with
| Success s -> switchFunction s
| Failure f -> Fa...

Bind as an adapter block
Two-track input Two-track output
let bind switchFunction =
fun twoTrackInput ->
match twoTrackInp...

Bind as an adapter block
Two-track input Two-track output
let bind switchFunction twoTrackInput =
match twoTrackInput with...

name50
Bind example
let nameNotBlank input =
if input.name = "" then
Failure "Name must not be blank"
else Success input
l...

Bind example
nameNotBlank (combined with)
name50 (combined with)
emailNotBlank
nameNotBlank name50 emailNotBlank

Bind example
bind nameNotBlank
bind name50
bind emailNotBlank
nameNotBlank name50 emailNotBlank

Bind example
bind nameNotBlank
>> bind name50
>> bind emailNotBlank
nameNotBlank name50 emailNotBlank

Bind example
let validateRequest =
bind nameNotBlank
>> bind name50
>> bind emailNotBlank
// validateRequest : TwoTrack<Re...

Bind example
let (>>=) twoTrackInput switchFunction =
bind switchFunction twoTrackInput
let validateRequest twoTrackInput ...

Bind doesn't stop transformations
FunctionB
type TwoTrack<'TEntity> =
| Success of 'TEntity
| Failure of string
FunctionA

Composing switches - review
Validate UpdateDb SendEmail
Validate UpdateDb SendEmail

Comic Interlude
What do you call a
train that eats toffee? I don't know, what do you
call a train that eats toffee?
A chew...

More fun with railway tracks...
Working with other functions

More fun with railway tracks...
Fitting other functions into this framework:
• Single track functions
• Dead-end functions...

Converting one-track functions
Fitting other functions into this framework:
• Single track functions
• Dead-end functions
...

Converting one-track functions
// trim spaces and lowercase
let canonicalizeEmail input =
{ input with email = input.email...

Converting one-track functions
UpdateDb SendEmailValidate
canonicalizeEmail
Won't compose

Converting one-track functions
Two-track input
Slot for one-track function
Two-track output

Converting one-track functions
Two-track input Two-track output
CanonicalizeCanonicalize

Converting one-track functions
Two-track input Two-track output
let map singleTrackFunction =
fun twoTrackInput ->
match t...

Converting one-track functions
Two-track input Two-track output
let map singleTrackFunction =
bind (singleTrackFunction >>...

Converting one-track functions
UpdateDb SendEmailValidate
canonicalizeEmail
Will compose

Converting dead-end functions
Fitting other functions into this framework:
• Single track functions
• Dead-end functions
•...

Converting dead-end functions
let updateDb request =
// do something
// return nothing at all
updateDb

Converting dead-end functions
SendEmailValidate UpdateDb
Won't compose

Converting dead-end functions
One-track input
Slot for dead end function
One-track output

Converting dead-end functions
One-track input One-track output
let tee deadEndFunction oneTrackInput =
deadEndFunction one...

Converting dead-end functions
SendEmailValidate
UpdateDb
Will compose

Functions that throw exceptions
Fitting other functions into this framework:
• Single track functions
• Dead-end functions...

Functions that throw exceptions
One-track input
Two-track output
SendEmail SendEmail
Add try/catch to
handle timeouts, say...

Functions that throw exceptions
EvenYoda recommends
not to use exception
handling for control flow:
Guideline: Convert exc...

Supervisory functions
Fitting other functions into this framework:
• Single track functions
• Dead-end functions
• Functio...

Supervisory functions
Two-track input Two-track output
Slot for one-track function for
Success case
Slot for one-track fun...

Putting it all together

Putting it all together
Validate
UpdateDb
SendEmail
Canonicalize
Input
Output??

Putting it all together
Validate
UpdateDb
SendEmail
Canonicalize
returnMessage
Input Output
let returnMessage result =
mat...

Putting it all together - review
The "two-track" framework is a useful approach for
most use-cases.
You can fit most funct...

Putting it all together - review
The "two-track" framework is a useful approach for
most use-cases.
let updateCustomer =
r...

Comic Interlude
Why can't a steam
locomotive sit down?
I don't know,
why can't a steam
locomotive sit down?
Because it
has...

Extending the framework:
• Designing for errors
• Parallel tracks
• Domain events

Designing for errors
Unhappy paths are requirements too

Designing for errors
let validateInput input =
if input.name = "" then
Failure "Name must not be blank"
else if input.emai...

Designing for errors
let validateInput input =
if input.name = "" then
Failure NameMustNotBeBlank
else if input.email = ""...

Designing for errors
let validateInput input =
if input.name = "" then
Failure NameMustNotBeBlank
else if input.email = ""...

Designing for errors
type ErrorMessage =
| NameMustNotBeBlank
| EmailMustNotBeBlank
| EmailNotValid of EmailAddress
// dat...

Designing for errors
type ErrorMessage =
| NameMustNotBeBlank
| EmailMustNotBeBlank
| EmailNotValid of EmailAddress
// dat...

Designing for errors – converting to strings
No longer works – each case must now
be explicitly converted to a string
retu...

Designing for errors – converting to strings
let returnMessage result =
match result with
| Success _ -> "Success"
| Failu...

Designing for errors - review
type ErrorMessage =
| NameMustNotBeBlank
| EmailMustNotBeBlank
| EmailNotValid of EmailAddre...

Parallel tracks

Parallel validation
nameNotBlank name50 emailNotBlank
Problem: Validation done in series.
So only one error at a time is r...

Parallel validation
nameNotBlank
name50
emailNotBlank
Split
input
Combine
output
Now we do get all
errors at once!
... But...

Combining switches
+
Trick: if we create an operation that combines pairs
into a new switch, we can repeat to combine as m...

Combining switches
+
+
Trick: if we create an operation that combines pairs
into a new switch, we can repeat to combine as...

Combining switches
+
+
Trick: if we create an operation that combines pairs
into a new switch, we can repeat to combine as...

Domain events
Communicating information to
downstream functions

Events are not errors
Validate
UpdateDb
SendEmail
Tell CRM that
email was sent

Events are not errors
Validate
UpdateDb
SendEmail
Tell CRM that
email was sent
type MyUseCaseMessage =
| NameMustNotBeBlan...

Events are not errors
Validate
UpdateDb
SendEmail
Tell CRM that
email was sent
type MyUseCaseMessage =
| NameMustNotBeBlan...

Comic Interlude
Why can't a train
driver be electrocuted? I don't know,
why can't a train driver
be electrocuted?
Because ...

Some topics not covered...
... but could be handled
in an obvious way.

Topics not covered
• Errors across service boundaries
• Async on success path (instead of sync)
• Compensating transaction...

Summary
A recipe for handling errors in a
functional way

Recipe for handling errors in a functional way
type TwoTrack<'TEntity> =
| Success of 'TEntity * Message list
| Failure of...

Demo
http://bit.ly/rop-example

I don’t always have errors...

I don’t always have errors...
Railway Oriented Programming
@ScottWlaschin
fsharpforfunandprofit.com
FPbridge.co.uk
http://...

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

Railway Oriented Programming

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