In an elm-pages app, each page can define a value data which is a DataSource that will be resolved before init is called. That means it is also available
when the page's HTML is pre-rendered during the build step. You can also access the resolved data in head to use it for the page's SEO meta tags.
A DataSource lets you pull in data from:
DataSource.File)DataSource.Http)content/*.txt (DataSource.Glob)DataSource.Port)DataSource.succeed "Hello!")DataSource.map2, DataSource.andThen, or other combining/continuing helpers from this moduleData from a DataSource is resolved when you load a page in the elm-pages dev server, or when you run elm-pages build.
Because elm-pages hydrates into a full Elm single-page app, it does need the data in order to initialize the Elm app.
So why not just get the data the old-fashioned way, with elm/http, for example?
A few reasons:
elm-pages has a build step, you know that your DataSource.Http requests succeeded, your decoders succeeded, your custom DataSource validations succeeded, and everything went smoothly. If something went wrong, you get a build failure and can deal with the issues before the site goes live. That means your users won't see those errors, and as a developer you don't need to handle those error cases in your code! Think of it as "parse, don't validate", but for your entire build.DataSource data. Also, it will be served up extremely quickly without needing to wait for any database queries to be performed, andThen requests to be resolved, etc., because all of that work and waiting was done at build-time!You can think of a DataSource as a declarative (not imperative) definition of data. It represents where to get the data from, and how to transform it (map, combine with other DataSources, etc.).
Even though an HTTP request is non-deterministic, you should think of it that way as much as possible with a DataSource because elm-pages will only perform a given DataSource.Http request once, and it will share the result between any other DataSource.Http requests that have the exact same URL, Method, Body, and Headers.
So calling a function to increment a counter on a server through an HTTP request would not be a good fit for a DataSource. Let's imagine we have an HTTP endpoint that gives these stateful results when called repeatedly:
https://my-api.example.com/increment-counter -> Returns 1 https://my-api.example.com/increment-counter -> Returns 2 https://my-api.example.com/increment-counter -> Returns 3
If we define a DataSource that hits that endpoint:
data =
DataSource.Http.get
"https://my-api.example.com/increment-counter"
Decode.intNo matter how many places we use that DataSource, its response will be "locked in" (let's say the response was 3, then every page would have the same value of 3 for that request).
So even though HTTP requests, JavaScript code, etc. can be non-deterministic, a DataSource always represents a single snapshot of a resource, and those values will be re-used as if they were a deterministic, declarative resource.
So it's best to use that mental model to avoid confusion.
Transform a request into an arbitrary value. The same underlying HTTP requests will be performed during the build step, but mapping allows you to change the resulting values by applying functions to the results.
A common use for this is to map your data into your elm-pages view:
import DataSource
import Json.Decode as Decode exposing (Decoder)
view =
DataSource.Http.get
(Secrets.succeed "https://api.github.com/repos/dillonkearns/elm-pages")
(Decode.field "stargazers_count" Decode.int)
|> DataSource.map
(\stars ->
{ view =
\model viewForPage ->
{ title = "Current stars: " ++ String.fromInt stars
, body = Html.text <| "⭐️ " ++ String.fromInt stars
, head = []
}
}
)This is useful for prototyping with some hardcoded data, or for having a view that doesn't have any StaticHttp data.
import DataSource
view :
List ( PagePath, Metadata )
->
{ path : PagePath
, frontmatter : Metadata
}
->
StaticHttp.Request
{ view : Model -> View -> { title : String, body : Html Msg }
, head : List (Head.Tag Pages.PathKey)
}
view siteMetadata page =
StaticHttp.succeed
{ view =
\model viewForPage ->
mainView model viewForPage
, head = head page.frontmatter
}Stop the StaticHttp chain with the given error message. If you reach a fail in your request,
you will get a build error. Or in the dev server, you will see the error message in an overlay in your browser (and in
the terminal).
Turn an Err into a DataSource failure.
Build off of the response from a previous DataSource request to build a follow-up request. You can use the data
from the previous response to build up the URL, headers, etc. that you send to the subsequent request.
import DataSource
import Json.Decode as Decode exposing (Decoder)
licenseData : DataSource String
licenseData =
DataSource.Http.get
(Secrets.succeed "https://api.github.com/repos/dillonkearns/elm-pages")
(Decode.at [ "license", "url" ] Decode.string)
|> DataSource.andThen
(\licenseUrl ->
DataSource.Http.get (Secrets.succeed licenseUrl) (Decode.field "description" Decode.string)
)Helper to remove an inner layer of Request wrapping.
Turn a list of StaticHttp.Requests into a single one.
import DataSource
import Json.Decode as Decode exposing (Decoder)
type alias Pokemon =
{ name : String
, sprite : String
}
pokemonDetailRequest : StaticHttp.Request (List Pokemon)
pokemonDetailRequest =
StaticHttp.get
(Secrets.succeed "https://pokeapi.co/api/v2/pokemon/?limit=3")
(Decode.field "results"
(Decode.list
(Decode.map2 Tuple.pair
(Decode.field "name" Decode.string)
(Decode.field "url" Decode.string)
|> Decode.map
(\( name, url ) ->
StaticHttp.get (Secrets.succeed url)
(Decode.at
[ "sprites", "front_default" ]
Decode.string
|> Decode.map (Pokemon name)
)
)
)
)
)
|> StaticHttp.andThen StaticHttp.combineA helper for combining DataSources in pipelines.
Like map, but it takes in two Requests.
view siteMetadata page =
StaticHttp.map2
(\elmPagesStars elmMarkdownStars ->
{ view =
\model viewForPage ->
{ title = "Repo Stargazers"
, body = starsView elmPagesStars elmMarkdownStars
}
, head = head elmPagesStars elmMarkdownStars
}
)
(get
(Secrets.succeed "https://api.github.com/repos/dillonkearns/elm-pages")
(Decode.field "stargazers_count" Decode.int)
)
(get
(Secrets.succeed "https://api.github.com/repos/dillonkearns/elm-markdown")
(Decode.field "stargazers_count" Decode.int)
)
A DataSource represents data that will be gathered at build time. Multiple
DataSources can be combined together using themapNfunctions, very similar to how you can manipulate values with Json Decoders in Elm.