Feeding a function result into a pattern matching function in F#

Say we have an F# function that returns a tuple of two elements:

let isGreaterThan x y =     
    if x > y then (true, x - y)
    else (false, 0)

…, i.e. we return true and the difference between the two input integers if the first integer is greater. Otherwise we return a false and a 0. Here’s how we can consume this function:

let isGreaterThan x y =     
    if x > y then (true, x - y)
    else (false, 0)
    
let (gt, diff) = isGreaterThan 10 6
printfn "Is greater: %b, diff: %i" gt diff

…which prints “Is greater: true, diff: 4” in the interactive window. We can feed the result into a pattern matching expression and branch our logic as follows:

isGreaterThan 19 6 |> function
    | (true, diff) -> printfn "Difference: %i" diff
    | (false, diff) -> printfn "Less then"

The pattern matching expression is introduced by the function keyword followed by pattern matching logic. The above program prints “Difference: 13” in the interactive window.

The pattern matching branches must be exhaustive so the following would not be sufficient:

isGreaterThan 19 6 |> function
    | (true, diff) -> printfn "Difference: %i" diff

The code still runs but we get a warning: Incomplete pattern matches on this expression. For example, the value ‘(false,_)’ may indicate a case not covered by the pattern(s).

We get an exception in case there’s no matching pattern branch:

isGreaterThan 6 19 |> function
    | (true, diff) -> printfn "Difference: %i" diff

Microsoft.FSharp.Core.MatchFailureException: The match cases were incomplete

View all F# related articles here.

Advertisements

About Andras Nemes
I'm a .NET/Java developer living and working in Stockholm, Sweden.

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google+ photo

You are commenting using your Google+ account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

ultimatemindsettoday

A great WordPress.com site

Elliot Balynn's Blog

A directory of wonderful thoughts

Robin Sedlaczek's Blog

Developer on Microsoft Technologies

Softwarearchitektur in der Praxis

Wissenswertes zu Webentwicklung, Domain-Driven Design und Microservices

Technology Talks

on Microsoft technologies, Web, Android and others

Software Engineering

Web development

Disparate Opinions

Various tidbits

chsakell's Blog

WEB APPLICATION DEVELOPMENT BEST PRACTICES WITH MICROSOFT STACK & ANGULAR

Cyber Matters

Bite-size insight on Cyber Security for the not too technical.

Guru N Guns's

OneSolution To dOTnET.

Johnny Zraiby

Measuring programming progress by lines of code is like measuring aircraft building progress by weight.

%d bloggers like this: