Make PostgREST requests in Elm
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Latest commit 5261145 Sep 18, 2018

README.md

PostgREST in Elm

This package creates a pleasant experience for interacting with PostgREST in Elm.

Design Goals

The design of the package is based on 2 central ideas:

  1. The PostgREST spec is not important to application authors. More generally, Transfer Protocols (HTTP) and Interchange Formats (JSON) are implementation details of an application. That's why in elm-postgrest, requests are thought of as "REST requests built from a schema" rather than "HTTP requests transfering JSON data".

  2. Invalid PostgREST requests should be compiler errors. HTTP errors caused by improperly constructed HTTP requests are, in many ways, equivalent to runtime errors. And, because this is Elm, you should not be able to generate runtime errors! That's why in elm-postgrest, where feasible, invalid requests are compiler errors.

Example

Let's take a look at an example. Say you have a PostgREST data server filled with NCES school data. You wish to fetch all of the schools' id and name fields where the school is in the state of California and then order them alphabetically. Let's take a look at how you'd build the request without this package:

-- without elm-postgrest
import Http exposing (Request)
import Json.Decode as Json exposing (Decoder)

type alias School =
    { id : String
    , name : String
    }

request : Request (List School)
request =
    Http.get "https://api.com/schools?select=id,name&state=eq.CA&order=asc.name" (Decode.list decoder)

decoder : Decoder School
decoder =
    map2 School
        (Json.field "id")
        (Json.field "name")

As demonstrated above, in the standard approach, you must build an HTTP GET request to the url https://api.com/schools?select=id,name&state=eq.CA&order=asc.name (Transfer Protocol) and decode the JSON response to your desired data structure (Interchange Format). Without elm-postgrest, the request code:

  1. contains inessential details of the PostgREST specification (ie. HTTP and JSON)
  2. is prone to invalid construction (because of heavy string usage)

Now, let's take a look at how you'd build the request with this package:

-- with elm-postgrest
import PostgRest as Rest exposing (Attribute, Request, Selection)
import Schema

type alias School =
    { id : String
    , name : String
    }

request : Request (List School)
request =
    Rest.readMany Schema.school
        { select = selection
        , where_ = Rest.eq "CA" .state
        , order = [ Rest.asc .name ]
        , limit = Nothing
        , offset = Nothing
        }

selection : Selection { a | id : Attribute String, name : Attribute String } School
selection =
    Rest.map2 School
        (Rest.field .id)
        (Rest.field .name)

As demonstrated above, this package leads to a more pleasant experience for interacting with PostgREST in Elm. With elm-postgrest, the request code:

  1. contains only the essential details of the data request
  2. is no longer prone to invalid construction (because the strings are gone)

As an aside, you may have noticed that the above example does not provide a definition for Schema.school. Dig into the docs to learn more!

Additional Resources