Elm Christmas

Once, twice, three times a value

A 2 minute read written by
Jørgen Tu Sveli
01.12.2020

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Welcome to the Elm christmas calendar, and to the first of 24 articles that will teach you some tricks and useful concepts you might not already know. Today we are discussing Tuples. It is perhaps Elm's simplest way of structuring data. We also introduce a concept recurring across several days; pattern matching, or destructuring. The first use-case is pattern matching of tuples in function declarations.

Tuples allow us to join values together into a single value. Elm, as of version 0.19, allows tuples of no more than 3 values. A function can receive a tuple as an argument or it can return one. The latter enables returning more than one value from a function. This requires more work in languages without tuples.

Here are som tuples:

governator : ( String, Int )
governator = ("Arnie", 73) -- A 2-Tuple

numbers : ( Int, Int, Int )
numbers = ( 12, 24, 18 ) -- A 3-tuple

The core module Tuple has helper functions for working with 2-tuples. E.g. Tuple.first and Tuple.second extract the respective values.

Tuple.first person   -- 23

The pattern of types of its values make up the tuple's type as a whole. Hence, a tuple of String and Int cannot serve as a tuple of Int and String.

Pattern matching tuples

In a function that takes a tuple, there are two main ways of "receiving" the argument. Receiving the tuple as a single value is the straightforward approach.

canDrive: (Bool, Bool) -> String
canDrive ageAndLicenseStatus =
  let 
    oldEnough = Tuple.first ageAndLicenseStatus
    hasLicense = Tuple.second ageAndLicenseStatus
  in
  if oldEnough && hasLicense then
    "Allowed to drive"
    
  else
    "Not allowed to drive"

Pattern matching the received tuple debloats the code considerably.

canDrive: (Bool, Bool) -> String
canDrive (oldEnough, hasLicense) =
  if oldEnough && hasLicense then
    "Allowed to drive"
    
  else
    "Not allowed to drive"

Pattern matching still works if you introduce a type alias for your tuple.

type alias DrivingRequirements = (Bool, Bool)

canDrive: DrivingRequirements -> String
canDrive (oldEnough, hasLicense) =
  -- ...

A look at core/Tuple

After establishing a basic understanding of a concept, I find it rewarding to look at API source code related to that concept. We saw how we could use Tuple.first to extract the first value from a tuple. Here is its source code.

first : (a, b) -> a
first (x,_) =
  x

Reading these type signatures can feel daunting at first. But let's apply what we know about tuples and how first works. Regarding the type annotation, (a, b) merely states that first takes a tuple of two values that might not be the same type. The return type is a, referring to the type of the first element of the incoming tuple. The type declaration is telling us: "I take a tuple of two values and return whatever type the tuple's first value had".

In line 2, we see that first makes use of pattern matching, albeit with a twist. first only needs the first value. The second value need not be named, but must be matched using the placeholder _.

Conclusion / TLDR

Elm has built-in syntax for creating tuples to structure data. When receiving a tuple as a parameter a similar syntax can be used to extract the tuple's values into separate variables. Heeding the notes in core/Tuple is advisable. When you need a little complexity in your data structure, tuples are rarely the right choice.

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