# How to use crul

#### Scott Chamberlain

#### 2017-03-31

The following aims to help you decide how to use `crul` in different scenarios.

First, `crul` is aimed a bit more at developers than at the casual user doing HTTP requests. That is, `crul` is probably a better fit for an R package developer, mainly because it heavily uses `R6` - an interface that’s very unlike the interface in `httr` but very similar to interacting with classes in Ruby/Python.

Second, there is not yet, but will be soon, the ability to mock HTTP requests. We are working on that, and should have it integrated soon. When that feature arrives we’ll update this vignette.

Load the library

    library("crul")

## A simple HTTP request function

Most likely you’ll want to do a `GET` request - so let’s start with that - though the details are not much different for other HTTP verbs.

And in most cases you’ll likely not want to do asynchronous requests - though see below if you do.

You’ll probably want to write a small function, like so (annotated for clarity)

    make_request <- function(url) {
      # create a HttpClient object, defining the url
      cli <- crul::HttpClient$new(url = url)
      # do a GET request
      res <- cli$get()
      # check to see if request failed or succeeded
      # - if succeeds this will return nothing and proceeds to next step
      res$raise_for_status()
      # parse response to plain text (JSON in this case) - most likely you'll
      # want UTF-8 encoding
      txt <- res$parse("UTF-8")
      # parse the JSON to an R list
      jsonlite::fromJSON(txt)
    }

Use the function

    make_request("https://httpbin.org/get")
    #> $args
    #> named list()
    #>
    #> $headers
    #> $headers$Accept
    #> [1] "*/*"
    #>
    #> $headers$`Accept-Encoding`
    #> [1] "gzip, deflate"
    #>
    #> $headers$Connection
    #> [1] "close"
    #>
    #> $headers$Host
    #> [1] "httpbin.org"
    #>
    #> $headers$`User-Agent`
    #> [1] "libcurl/7.51.0 r-curl/2.4 crul/0.3.4"
    #>
    #>
    #> $origin
    #> [1] "46.189.67.31"
    #>
    #> $url
    #> [1] "https://httpbin.org/get"

Now you can use the `make_request` function in your script or package.

## More customized function

Once you get more familiar (or if you’re already familiar with HTTP) you may want to have more control, toggle more switches.

In the next function, we’ll allow for users to pass in curl options, use a custom HTTP status checker, and xxx.

    make_request2 <- function(url, ...) {
      # create a HttpClient object, defining the url
      cli <- crul::HttpClient$new(url = url)
      # do a GET request, allow curl options to be passed in
      res <- cli$get(...)
      # check to see if request failed or succeeded
      # - a custom approach this time combining status code,
      #   explanation of the code, and message from the server
      if (res$status_code > 201) {
        mssg <- jsonlite::fromJSON(res$parse("UTF-8"))$message$message
        x <- res$status_http()
        stop(
          sprintf("HTTP (%s) - %s\n  %s", x$status_code, x$explanation, mssg),
          call. = FALSE
        )
      }
      # parse response
      txt <- res$parse("UTF-8")
      # parse the JSON to an R list
      jsonlite::fromJSON(txt)
    }

Use the function

    make_request2("http://api.crossref.org/works?rows=0")
    #> $status
    #> [1] "ok"
    #>
    #> $`message-type`
    #> [1] "work-list"
    #>
    #> $`message-version`
    #> [1] "1.0.0"
    #>
    #> $message
    #> $message$facets
    #> named list()
    #>
    #> $message$`total-results`
    #> [1] 87613771
    #>
    #> $message$items
    #> list()
    #>
    #> $message$`items-per-page`
    #> [1] 0
    #>
    #> $message$query
    #> $message$query$`start-index`
    #> [1] 0
    #>
    #> $message$query$`search-terms`
    #> NULL

No different from the first function (besides the URL). However, now we can pass in curl options:

    make_request2("http://api.crossref.org/works?rows=0", verbose = TRUE)
    make_request2("http://api.crossref.org/works?rows=0", timeout_ms = 1)

We can also pass named parameters supported in the `get` method, including `query`, `disk`, and `stream`.

    make_request2("http://api.crossref.org/works", query = list(rows = 0))
    #> $status
    #> [1] "ok"
    #>
    #> $`message-type`
    #> [1] "work-list"
    #>
    #> $`message-version`
    #> [1] "1.0.0"
    #>
    #> $message
    #> $message$facets
    #> named list()
    #>
    #> $message$`total-results`
    #> [1] 87613771
    #>
    #> $message$items
    #> list()
    #>
    #> $message$`items-per-page`
    #> [1] 0
    #>
    #> $message$query
    #> $message$query$`start-index`
    #> [1] 0
    #>
    #> $message$query$`search-terms`
    #> NULL

In addition, the failure behavior is different, and customized to the specific web resource we are working with

    make_request2("http://api.crossref.org/works?rows=asdf")
    #> Error: HTTP (400) - Bad request syntax or unsupported method
    #>   Integer specified as asdf but must be a positive integer less than or equal to 1000.

## Asynchronous requests

You may want to use asynchronous HTTP requests when any one HTTP request takes “too long”. This is of course all relative. You may be dealing with a server that responds very slowly, or other circumstances.

See the async with crul vignette for more details on asynchronous requests.