The Maybe value in Elm is useful when parsing a String into a floating point number, or when requesting data that may not exist from a server. The response data in an Elm app is decoded by an Elm application and the value can then be handled with relative certainty.
This example of a Maybe value comes from the Elm documentation.
Elm in the terminal ↓
String.toFloat
<function> : String -> Maybe Float
> String.toFloat “3.1415”
Just 3.1415 : Maybe Float
> String.toFloat “abc”
Nothing : Maybe FloatThe string given to toFloat may not be a valid number, in which case the output of the toFloat method could be represented by some failure but to say the output is Maybe a float value makes sense.
A Maybe value can be something or nothing, where both something and nothing cases must be described - generally by using a case statement - which is the same as a JS switch statement.
Elm ↓
let maybeFloat =
String.toFloat “3.1415”
let usefulValue =
case maybeFloat of
Just floatValue ->
floatValue
Nothing ->
1.0This case statement says that in a case where the value is nothing a default value of 1.0 is provided. Default values should be meaningful and should make sense.
Elm ↓
type Maybe a
= Just a
| NothingBelow is an example of some metadata from a markdown blog post. Values brought into an Elm app require decoding into Elm type values.
Markdown blog post
Markdown ↓
---
{
slug: 'maybe-decoders-and-updaters-elm',
title: 'Maybes, Decoders and updaters in elm',
tags: ['web'],
publishDate: '2018-09-12',
description: 'This is a blog about Maybe values.',
}
---
This is a post about Maybe values.Elm decoder
Elm ↓
-- import Decoder as Decode exposing (Decoder)
type Decoder a
type alias BlogPostMetadata =
{ slug : String
, title : String
, tags : List String
, publishDate : IsoString
, description : String
}
blogPostMetaDecoder : Decoder BlogPostMetadata
blogPostMetaDecoder =
Decode.map5 BlogPostMetadata
(Decode.field "slug" Decode.string)
(Decode.field "title" Decode.string)
(Decode.field "tags" (Decode.list Decode.string))
(Decode.field "publishDate" Decode.string)
(Decode.field "description" Decode.string)A Decoder wraps data coming into an Elm application in a Functor and then tries to translate that data into a valid Elm type value.
BlogPostMetadata is a constructor that defines metadata from the markdown file above.
The markdown data is wrapped in a Decoder called blogPostMetaDecoder, which expects certain field values. Given a field name and a decoder, the Decoder should translate the data into an expected Elm value.
Each field decoder - Decode.field - is separate so the top level decoder performs a transformation called map5 which takes 5 Decoder values and returns just one Decoder value. map is an important feature of both Maybe and Decoder values.
The Decoder type definition looks similar to the Just branch in a Maybe type.
A decoder and a Maybe value perform different functions but each one wraps a value in a functor. This functor is a type of monad.
A monad Functor has a map method and a bind method. A wrapped Maybe or Decode value can be transformed using the map method available on the monad.
It may also handle computations that return nested wrapped values - by using a bind method. bind exists in Elm on Maybe and Decode values as andThen.
Before learning Elm I wrote a lot of JavaScript and found the Maybe.map method confusing as I had only seen map used on arrays. The map method in each case does much the same thing.
Elm ↓
-- Maybe
map : (a -> b) -> Maybe a -> Maybe b
andThen : (a -> Maybe b) -> Maybe a -> Maybe b
-- Decoders
map : (a -> value) -> Decoder a -> Decoder value
andThen : (a -> Decoder b) -> Decoder a -> Decoder b
-- Also Decoders. Helpful for dealing with optional fields.
maybe : Decoder a -> Decoder (Maybe a)The type signatures in each case are the same apart from the functor wrapping the given value.
Definitions
map -> Given a transformation and a functor-wrapped value,
output a transformed value wrapped in a functor.
andThen -> Given a transformation and a functor-wrapped value,
escape or flatten the first functor and output another wrapped value.
Elm decoder with an optional field
In the example below, the authorName value may not be included in the blog post metadata so in the UI an author’s name may not be displayed. This value is therefore wrapped in both a Decoder functor and also wrapped in a Maybe value functor and will need to be unwrapped from both Decoder and Maybe.
Elm ↓
-- import Decoder as Decode exposing (Decoder)
type Decoder a
type alias BlogPostMetadata =
{ slug : String
, title : String
, tags : List String
, publishDate : IsoString
, description : String
-- authorName is an optional field
, authorName : Maybe String
}
blogPostMetaDecoder : Decoder BlogPostMetadata
blogPostMetaDecoder =
Decode.map5 BlogPostMetadata
(Decode.field "slug" Decode.string)
(Decode.field "title" Decode.string)
(Decode.field "tags" (Decode.list Decode.string))
(Decode.field "publishDate" Decode.string)
(Decode.field "description" Decode.string)
-- Their may not be an author field to decode
(Decode.maybe (Decode.field "authorName" Decode.string))The authorName field is optional and there is no meaningful default value.
In this particular case Decode.maybe is well suited to decoding the field, though if there was a meaningful default value a package like Json-Decode-Pipeline could be used to neatly handle a missing value.
https://package.elm-lang.org/packages/elm/json/latest/Json-Decode#maybe
Summary
My reason for writing this article was because I thought it would be useful to understand monads when using Elm, as it is a functional language. I found that understanding monads in great detail is not strictly necessary, however the journey to understanding has prompted me to ask questions at work and read many things. This post became far more practical than I expected.