# Neturl_tut

## Neturl Tutorial

This module is a quite flexible parser for various kinds of URLs occuring in practice. The syntax is configurable such that one URL module can handle a lot of URL types in a generic way.

### Generic Parsing

In order to parse an absolute URL (beginning with a scheme identifier like "http:...") of unknown type just call

    let url = parse_url "http://me@server/directory"

By default, this function can parse all URL types listed at `Neturl.common_url_syntax`. However, the default configuration implies also that

- relative URLs cannot be parsed
- fragment identifiers are rejected (i.e. the part after the hash mark like in "http://server/document#location")
- characters are rejected when the most significant bit (MSB) is set

The latter two features can be simply enabled by passing the arguments `~enable_fragment:true` and `~accept_8bits:true`, respectively.

The restriction that relative URLs are rejected has to do with the problem that context information is missing. Because the scheme identifier (like "http") is not available, the function does not know which syntax the relative URL should have. For example, the relative URL `dir/file?x=1` is differently parsed when it is taken relative to an `http` URL and when it is interpreted relative to an `ftp` URL. In the first case, the path component of the URL is `"dir/file"` and the query component is `"?x=1"`, but in the latter case the path component is `"dir/file?x=1"`, and a query component is not allowed.

The solution is that the syntax of the base URL, relative to which the URL is seen, must be passed as additional argument. Under the assumption that `base_url` is the base URL, use

    let url = parse_url
                ~base_syntax:(url_syntax_of_url base_url)
                "/dir/file?x=1"

Of course, this assumes that the base URL is known when the url is parsed.

### Parsing For a Certain Syntax

The function `url_of_string` is also a parser, but you must pass the URL syntax as argument, e.g.

    let url = url_of_string syntax "ipp://server/printer"

Pass as `syntax` one of the elements of `Neturl.common_url_syntax`, e.g.

     let syntax = Hashtbl.find common_url_syntax "ipp"

or a self-defined syntax.

### Printing URLs

This is much easier, just call `string_of_url` to convert an URL to a string. It is ensured that every URL always has an unambiguous representation as string.

### URL Components

Internally, the parsed URL is decomposed into its components. This module supports two ways of decomposition:

1. `scheme://user;userparams:password@host:port/path;params?query#fragment`
2. `scheme:other;params?query#fragment`

The first form is used for services that directly connect to a certain service running on a certain host. The second form can be used for everything else not falling under this category.

Examples:

- `http://me:abrakadabra@server/dir?x=5#section1` scheme=`"http"`, user=`"me"`, password=`"abrakadabra"`, host=`"server"`, path=`"/dir"`, query=`"x=5"`, fragment=`"section1"`
- `pop://you;auth=digest-md5@mail` scheme=`"pop"`, user=`"you"`, user_params=`["auth=digest-md5"]`, host=`"mail"`
- `mailto:gerd@gerd-stolpmann.de?cc=you@domain.com` scheme=`"mailto"`, other=`"gerd@gerd-stolpmann.de"`, query=`"cc=you@domain.com"`

It is important to mention that the decomposition is not fully performed, but only down to a certain level. For example, the query `"x=5"` could be further analysed and be split into the syntactic parts `"x"` and `"5"`. However, this is not done, just because the author seeked a compromise between the depth of analysis and the genericy of application.

### URL Escaping

In order to represent the so-called unsafe characters, one can use `%`-escaping in URLs. For example, this URL contains a password with `@`, an unsafe character encoded as `%40`:

     http://user:!$%40?@server

The question is how this module handles such escapings.

It is surprising that the URL parser does not decode these escaped forms (it checks, however, whether they are syntactically correct). Internally, the components are stored as parsed, and one can even retrieve them in their original form. The function `url_password` returns the password component. Applied to the above URL, one can get the password in its original, "encoded" form, or as decoded string:

- `url_password ~encoded:true url` returns `"!$%40?"`
- `url_password url` returns `"!$@?"`

### Representation of URL Components

The URL components can be retrieved with the functions

- `url_scheme`
- `url_user`
- `url_user_param`
- `url_password`
- `url_host`
- `url_port`
- `url_path`
- `url_param`
- `url_query`
- `url_fragment`
- `url_other`

Most components are just strings. Of course, the port number is an integer.

The path component (`url_path`) has a non-obvious representation. The path is represented as string list, e.g. "a/b/c" is represented as `["a";"b";"c"]` . Note, however, that absolute paths have an empty string at the beginning of the list, e.g. "/a/b/" is `[""; "a"; "b"; "" ]` . In most cases, the paths found in URLs are absolute, and because of this it is quite common to find this empty string at the beginning of the path list. The corner cases are:

- `[]` is used when the path is missing in the URL
- `[ "" ]` is "/"
- `[ ""; "" ]` is considered as illegal

The last two cases are somewhat arbitrary.

There is the helper function `split_path` to convert the string representation of paths into the list representation.

The parameters (`url_user_param` and `url_param`) are lists, too. A parameter starts with a semicolon as delimiter and runs until the next component, which can be another parameter. The contents, i.e. the values after the semicolons are put into the list. For example, the parameter ";auth=unix;type=i" is represented as `["auth=unix"; "type=i"]` .

### Hint: Getting Query Arguments

The query component is represented as a single string. When queries use the standard syntax "name1=value1&name2=value2&...", one can parse this string using

    let args = Netencoding.Url.dest_url_encoded_parameters
                 (url_query ~encoded:true url)

Note that `encoded:true` is needed.

### Creating and Modifying URLs

In order to create a URL for a certain syntax, call `make_url`:

    let url = make_url
                ~scheme:"http"
                ~user:"user"
                ~password:"!$@?"
                ~host:"server"
                syntax

It is checked whether the URL conforms to the passed syntax. By default, the components are passed in decoded form, and `make_url` automatically encodes them if necessary (here, for example, the at sign in the password). Alternatively, one can set `~encoded:true`, and pass the already escaped components. In this case, `make_url` checks whether the encoding is sufficient to represent the URL as string.

The functions `modify_url`, `default_url`, `undefault_url`, and `remove_from_url` can be used to modify an existing URL.

### Relative URLs

A URL is relative when the scheme identifier at the beginning is omitted. In this case, the URL can be transformed to an absolute URL when the base URL is known. The algorithm for this is defined in RFC 1808, and quite complicated. It is implemented in `apply_relative_url`, but usually `Neturl.ensure_absolute_url` is the more convenient function. Just call

     let url' = ensure_absolute_url ~base url

to convert `url` to its absolute counterpart `url'` when it is relative, and to pass the URL unchanged when it is already absolute.