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elm-data: Experimental implementation of generic operations for elm. Build Status

Provides means for defining resusable code that can be applied to a List, a Maybe, or any other data structure you provide the base functions for.

This library is experimental in nature. Whilst it should be fairly safe to use, I can only really recomend using this library for trying out some of the operations in elm-repl and saying "huh", or maybe smaller projects. I made it to try and test what's possible with restrictions present in elm, and the answer is rather a lot.

An overview of the main restriction is a little jargony and included at the end of this page.

Overview of operations

This library provides the data modules Foldable, Functor and Traversable, and the control modules Applicative and Monad. A bunch of default definitions for List, Maybe, Result and other types are provided.

If you don't know what one of the later modules is, I'd recommend learning about it elsewhere, but an overview of what's possible in each module is provided below.

Folding things

Folding a list to get the total sum of elements is very convenient.

List.sum [1,2,3,4] == 10

But what if you could define this same sum function once and apply it to arrays, sets, results,..? With this library, you can do exactly that:

sum list [1,2,3,4] == 10
sum maybe (Just 10) == 10
sum array (fromList [1,2,9]) == 12

Thus a type can belong to a class of types that are suitable for a given operation. In this case things of type List Int or Array Int are perfectly valid arguments to sum, as well as anything else you can reasonably define foldr for.

sum { foldr } = foldr (+) 0

This is saying "Give me a record containing the function foldr for a container, and I'll show you how to add it up." This is the basis for the entire library.

Notice that the function doesn't take a complicated union type or anything. That's because this library strives for convenience, and being able to define functions that only use exactly the operations they need helps with this immensely.


Another convenient operation on lists is mapping. ((+) 2) [10,9,8,7] == [12,11,10,9]

A structure that can be mapped over like this can be called a functor. So the following map operation works for any given functor.

.map list ((+) 2) [10,9,8,7] == [12,11,10,9]
.map maybe ((*) 345) Nothing == Nothing

The dot . in .map is necessary because this is actually just a field in a record. This hints at how the instance is defined, which is something like this:

list = { map = }

DISCLAIMER: The explanations will be less explanatory from here, this library shouldn't be your first intro to these topics.


An Applicative is like a functor but the function to apply can itself be inside a structure.

.andMap list [((*) 2), ((/) 4)] [4,5,10] == [8,10,20,1,0.8,0.4]
.andMap ziplist [((*) 2), ((/) 4)] [4,5,10] == [8,0.8] 

A cool feature of this is that you can change the semantics by simply changing the definitions in the record, as evidenced by the ziplist example. Having said that, this ziplist isn't technically a valid applicative, the short reason being that in elm the default lists aren't lazy.


Traversing is very useful, it allows you to take a container of effects and collect them together. This is much better explained by examples:

sequence Traversable.list Applicative.maybe [Just 4, Just 10] == Just [4,10]
sequence Traversable.list Applicative.maybe [Just 4, Nothing] == Nothing

.traverse Traversable.list Applicative.maybe List.head [[]] == Nothing

sequence Traversable.list Applicative.list [[1,2,3],[4,5,6]] ==

We start needing to be more explicit about which definitions of things we want here, but it makes things more readable for the person trying to understand it at least.

I think the best example of this is with Result.

sequence Traversable.list Applicative.result [Ok 3, Err "Oh dear", Ok 20] == Err "Oh dear"
sequence Traversable.list Applicative.result [Ok 3, Ok 40] == Ok [3, 40]

If you have some function that applies across a list of values and may fail, it's a great time to traverse with that function. You'll either get back a list containing your succesful results, or a single error message.


I've been avoiding types up to here, because they are not as pretty as the code. However, I'll assume that if you got this far you already have some understanding of the underlying topics, so I'll show you what the story is like on the type level.

I don't know what the hell this is, but it's certainly not a monad:

type alias Monad a b c d e f g h i j k l =
    AndThenable a b c d (AndMappable e f g h i (Mappable j k l {}))

But, save the restriction below, it works. The code itself can still be very neat and readable.

Here is a definition of join. That is, concat but for an arbitrary monad.

join : { d | andThen : (a -> a) -> b -> c } -> b -> c
join { andThen } m = m |> andThen identity

join list [[1,2],[4,5,6]] == [1,2,4,5,6]
join maybe (Just Nothing) == Nothing

It's clear here that the type is too general for it's own good, but there's not much that can be done about it. As soon as you apply it to a record though, you get something which starts to look more reasonable.

> join list
<function> : List (List b) -> List b

Although this is of course dependent on how well the andThen function is defined. The Monad alias (along with the other aliases) help to aleviate this somewhat, but ultimately there is a greater risk of problems and bad definitions than there would be if the type came out like this:

join : { d | andThen : (a -> m b) -> m a -> m b } -> m (m a) -> m a

Hence the inclusion of unit tests.

Main restriction

Jargon warning

Due to the way that type inference works, these classes do not have their full power that they might have in a higher kinded language with rank 2 polymorphism.

This is a complicated way of saying that when the function is passed in, the types are locked with no way of saying that they can alter, and so this will fail to compile:

compute { andThen, pure } a = pure a |> andThen (List.head) |> andThen (pure << ((*) 2))

Whereas a version that explicitly uses maybe will work out just fine:

compute a = Maybe.Just a |> Maybe.andThen (List.head) |> Maybe.andThen (Maybe.Just << ((*) 2))

There is an issue pertaining to the addition of ranked polymorphism here if you want more information.

However, just because you can't perform chaining like this doesn't mean that you can't define some very neat and powerful abstractions! For example, foldM is definable as it does not require chaining andThen, and is defined in Control.Monad.

Why did you make this?

Mostly for the "Is that possible?" factor, but also because I find that operations like these can reduce code duplication immensely. Hopefully one day elm will support something more robust natively, but for the time being this is the best alternative I have found.