form.md
In rvest: Easily Harvest (Scrape) Web Pages

has useful print method

Code
  html_form(html, base_url = "http://google.com")[[1]]
Output
  <form> 'test' (POST http://google.com/test-path)
    <field> (select) select: 
    <field> (text) name: Hadley
    <field> (password) name: ******
    <field> (button) clickMe: 
    <field> (textarea) address: ABCDEF

Code
  html_form(html)[[1]]$fields[[2]]
Output
  <field> (text) name: Hadley

handles different encoding types

Code
  convert_enctype("unknown")
Condition
  Warning:
  Unknown enctype (unknown). Defaulting to form encoded.
Output
  [1] "form"

validates its inputs

Code
  html_form(html_element(select, "button"))
Condition
  Error in `html_form()`:
  ! `x` must be a <form> element.
Code
  html_form(select, base_url = 1)
Condition
  Error in `FUN()`:
  ! `base_url` must be a single string or `NULL`, not the number 1.

can set values of inputs

Code
  form <- html_form_set(form, hidden = "abc")
Condition
  Warning:
  Setting value of hidden field "hidden".

has informative errors

Code
  html_form_set(form, text = "x")
Condition
  Error in `html_form_set()`:
  ! Can't change value of input with type submit: "text".

Code
  html_form_set(form, missing = "x")
Condition
  Error in `html_form_set()`:
  ! Can't set value of fields that don't exist: "missing".

useful feedback on invalid forms

Code
  submission_build(form, NULL)
Condition
  Error:
  ! `form` doesn't contain a `action` attribute.

Code
  x <- submission_build(form, NULL)
Condition
  Warning:
  Invalid method (FOO), defaulting to GET.

handles multiple buttons

Code
  vals <- submission_build_values(form, NULL)
Message
  Submitting with button "one".

Code
  submission_build_values(form, 3L)
Condition
  Error:
  ! Numeric `submit` out of range.

Code
  submission_build_values(form, "three")
Condition
  Error:
  ! No <input> found with name "three".
  i Possible values: "one" and "two".

Code
  submission_build_values(form, TRUE)
Condition
  Error:
  ! `submit` must be NULL, a string, or a number.

can submit using three primary techniques

Code
  show_response(html_form_submit(form))
Output
  GET 
  Query string: x=1&x=2&y=3

Code
  form$method <- "POST"
  show_response(html_form_submit(form))
Output
  POST application/x-www-form-urlencoded
  Query string: 
  x=1&x=2&y=3
Code
  form$enctype <- "multipart"
  show_response(html_form_submit(form))
Output
  POST multipart/form-data; boundary=---{divider}
  Query string: 
  ---{divider}
  Content-Disposition: form-data; name="x"

  1
  ---{divider}
  Content-Disposition: form-data; name="x"

  2
  ---{divider}
  Content-Disposition: form-data; name="y"

  3
  ---{divider}