# HTML output from queries In addition to output generated using write/1, format/2, print_message/2, etc., SWISH allows for HTML output. This is arranged using a wrapper around library(http/html_write) that is used for generating HTML output from the web server framework. Generating HTML is presented to the user using the predicate html/1. A general tutorial for producing HTML using this approach has been written by Anne Ogborn and can be found [here](https://github.com/Anniepoo/swiplwebtut/blob/master/web.adoc#html1-and-termerized-html) The html/1 predicate can also be used from [evaluable code in markdown cells](example/md_eval.swinb) The argument to html/1 is one of - __An atom or string__ is processed as text. Note that HTML special characters are escaped by the library. - __A list__ is processed element-by-element. - __A term Element(Content)__ creates ``<Element>Content</Element>`` - __A term Element(Attributes, Content)__ creates an element as above with _attributes_. `Attributes` takes one of shapes below, where Value is _atomic_ or a term `A+B` that results in the concatenation of `A` and `B`. - __A term Name(Value)__ - __A term Name=Value__ - __A list__ of attribute terms. - __A term \Rule__ calls the DCG `Rule` to produce more HTML. `Rule` typically calls (indirectly) html//1. - __A term \List__ inserts the elements of `List` unchecked into the HTML output. __THIS IS DISCOURAGED AS IT MAY CREATE INVALID HTML__. This notebook gives some examples. We start with the common _hello world_ example.

## Calling rules The example below introduces both element attributes and the ability to call _rules_ from html/1. In this case our rule is ``dice//1``. The ability to call rules allows for creating __reusable__ fragments.

## Using high order DCG The HTML is constructed using _non-terminals_, also known as _DCG rules_, written as `Head --> Body`. The library(dcg/high_order) provides some high order primitives that allows you to abstract from loops much the same way as the _maplist_ family (see maplist/3). In the examples below we use the foreach//2 constructs that creates a sequence from all solutions of the first argument. We start with a trivial example.

The next example shows this construct in a more advanced setting. It illustrates that you can create stylished output in a single call. In practice is it probably better style to write a predicate to generate a single row, similar to the `dice` example above.

## Inserting literal HTML The _quasi quotation_ mechanism of SWI-Prolog allows for inserting literal HTML strings and interpolating values. Note that the interpolated Prolog value must appear as the entire content or attribute value of the HTML input. Below is an example.