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How to use crul
===============

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


```r
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)


```r
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


```r
make_request("https://httpbin.org/get")
#> $args
#> named list()
#> 
#> $headers
#> $headers$Accept
#> [1] "application/json, text/xml, application/xml, */*"
#> 
#> $headers$`Accept-Encoding`
#> [1] "gzip, deflate"
#> 
#> $headers$Connection
#> [1] "close"
#> 
#> $headers$Host
#> [1] "httpbin.org"
#> 
#> $headers$`User-Agent`
#> [1] "libcurl/7.54.0 r-curl/3.1 crul/0.5.0"
#> 
#> 
#> $origin
#> [1] "50.22.155.214"
#> 
#> $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.


```r
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


```r
make_request2("https://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] 94347210
#> 
#> $message$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:


```r
make_request2("https://api.crossref.org/works?rows=0", verbose = TRUE)
make_request2("https://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`.


```r
make_request2("https://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] 94347210
#> 
#> $message$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


```r
make_request2("https://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.
