# (lispkit markdown)
Library `(lispkit markdown)` provides an API for programmatically constructing [Markdown](https://daringfireball.net/projects/markdown/) documents, for parsing strings in Markdown format, as well as for mapping Markdown documents into corresponding HTML. The Markdown syntax supported by this library is based on the [CommonMark Markdown](https://commonmark.org) specification.
## Data Model
Markdown documents are represented using an abstract syntax that is implemented by three algebraic datatypes `block`, `list-item`, and `inline`, via `define-datatype` of library `(lispkit datatype)`.
### Blocks
At the top-level, a Markdown document consist of a list of *blocks*. The following recursively defined datatype shows all the supported block types as variants of type `block`.
```scheme
(define-datatype block markdown-block?
(document blocks)
where (markdown-blocks? blocks)
(blockquote blocks)
where (markdown-blocks? blocks)
(list-items start tight items)
where (and (opt fixnum? start) (markdown-list? items))
(paragraph text)
where (markdown-text? text)
(heading level text)
where (and (fixnum? level) (markdown-text? text))
(indented-code lines)
where (every? string? lines)
(fenced-code lang lines)
where (and (opt string? lang) (every? string? lines))
(html-block lines)
where (every? string? lines)
(reference-def label dest title)
where (and (string? label) (string? dest) (every? string? title))
(table header alignments rows)
where (and (every? markdown-text? header)
(every? symbol? alignments)
(every? (lambda (x) (every? markdown-text? x)) rows))
(definition-list defs)
where (every? (lambda (x)
(and (markdown-text? (car x))
(markdown-list? (cdr x)))) defs)
(thematic-break))
```
`(document blocks)` represents a full Markdown document consisting of a list of blocks. `(blockquote blocks)` represents a blockquote block which itself has a list of sub-blocks. `(list-items start tight items)` defines either a bullet list or an ordered list. *start* is `#f` for bullet lists and defines the first item number for ordered lists. *tight* is a boolean which is `#f` if this is a loose list (with vertical spacing between the list items). *items* is a list of list items of type `list-item` as defined as follows:
```scheme
(define-datatype list-item markdown-list-item?
(bullet ch tight? blocks)
where (and (char? ch) (markdown-blocks? blocks))
(ordered num ch tight? blocks)
where (and (fixnum? num) (char? ch) (markdown-blocks? blocks)))
```
The most frequent Markdown block type is a paragraph. `(paragraph text)` represents a single paragraph of text where *text* refers to a list of inline text fragments of type `inline` (see below). `(heading level text)` defines a heading block for a heading of a given level, where *level* is a number starting with 1 (up to 6). `(indented-code lines)` represents a code block consisting of a list of text lines each represented by a string. `(fenced-code lang lines)` is similar: it defines a code block with code expressed in the given language *lang*. `(html lines)` defines a HTML block consisting of the given lines of text. `(reference-def label dest title)` introduces a reference definition consisting of a given *label*, a destination URI *dest*, as well as a *title* string. `(table header alignments rows)` defines a table consisting of *headers*, a list of markdown text describing the header of each column, *alignments*, a list of symbols `l` (= left), `c` (= center), and `r` (= right), and *rows*, a list of lists of markdown text. `(definition-list defs)` represents a definition list where *defs* refers to a list of definitions. A definition has the form `(name def ...)` where *name* is markdown text defining a name, and *def* is a bullet item using `:` as bullet character. Finally, `(thematic-break)` introduces a thematic break block separating the previous and following blocks visually, often via a line.
### Inline Text
Markdown text is represented as lists of inline text segments, each represented as an object of type `inline`. `inline` is defined as follows:
```scheme
(define-datatype inline markdown-inline?
(text str)
where (string? str)
(code str)
where (string? str)
(emph text)
where (markdown-text? text)
(strong text)
where (markdown-text? text)
(link text uri title)
where (and (markdown-text? text) (string? uri) (string? title))
(auto-link uri)
where (string? uri)
(email-auto-link email)
where (string? uri)
(image text uri title)
where (and (markdown-text? text) (string? uri) (string? title))
(html tag)
where (string? tag)
(line-break hard?))
```
`(text str)` refers to a text segment consisting of string *str*. `(code str)` refers to a code string *str* (often displayed as verbatim text). `(emph text)` represents emphasized *text* (often displayed as italics). `(strong text)` represents *text* in boldface. `(link text uri title)` represents a hyperlink with *text* linking to *uri* and *title* representing a title for the link. `(auto-link uri)` is a link where *uri* is both the text and the destination URI. `(email-auto-link email)` is a "mailto:" link to the given email address *email*. `(image text uri title)` inserts an image at *uri* with image description *text* and image link title *title*. `(html tag)` represents a single HTML tag of the form `<`*tag*`>`. Finally, `(line-break #f)` introduces a "soft line break", whereas `(line-break #t)` inserts a "hard line break".
## Creating Markdown documents
Markdown documents can either be constructed programmatically via the datatypes introduced above, or a string representing a Markdown documents gets parsed into the internal abstract syntax representation via function `markdown`.
For instance, `(markdown "# My title\n\nThis is a paragraph.")` returns a markdown document consisting of two blocks: a *header block* for header "My title" and a *paragraph block* for the text "This is a paragraph":
```scheme
(markdown "# My title\n\nThis is a paragraph.")
⟹ #block:(document (#block:(heading 1 (#inline:(text "My title"))) #block:(paragraph (#inline:(text "This is a paragraph.")))))
```
The same document can be created programmatically in the following way:
```scheme
(document
(list
(heading 1 (list (text "My title")))
(paragraph (list (text "This is a paragraph.")))))
⟹ #block:(document (#block:(heading 1 (#inline:(text "My title"))) #block:(paragraph (#inline:(text "This is a paragraph.")))))
```
## Processing Markdown documents
Since the abstract syntax of Markdown documents is represented via algebraic datatypes, pattern matching can be used to deconstruct the data. For instance, the following function returns all the top-level headers of a given Markdown document:
```scheme
(import (lispkit datatype)) ; this is needed to import `match`
(define (top-headings doc)
(match doc
((document blocks)
(filter-map (lambda (block)
(match block
((heading 1 text) (text->raw-string text))
(else #f)))
blocks))))
```
An example for how `top-headings` can be applied to this Markdown document:
```markdown
# *header* 1
Paragraph.
# __header__ 2
## header 3
The end.
```
is shown here:
```scheme
(top-headings (markdown "# *header* 1\nParagraph.\n# __header__ 2\n## header 3\nThe end."))
⟹ ("header 1" "header 2")
```
## API
**block-type-tag** 
Symbol representing the markdown `block` type. The `type-for` procedure of library `(lispkit type)` returns this symbol for all block objects.
**list-item-type-tag**
Symbol representing the markdown `list-item` type. The `type-for` procedure of library `(lispkit type)` returns this symbol for all list item objects.
**inline-type-tag**
Symbol representing the markdown `inline` type. The `type-for` procedure of library `(lispkit type)` returns this symbol for all inline objects.
**(markdown-blocks? *****obj*****)** 
Returns `#t` if *obj* is a proper list of objects *o* for which `(markdown-block?`` `*`o`*`)` returns `#t`; otherwise it returns `#f`.
**(markdown-block? *****obj*****)**
Returns `#t` if *obj* is a variant of algebraic datatype `block`.
**(markdown-block=? *****lhs rhs*****)**
Returns `#t` if markdown blocks *lhs* and *rhs* are equals; otherwise it returns `#f`.
**(markdown-list? *****obj*****)**
Returns `#t` if *obj* is a proper list of list items *i* for which `(markdown-list-item?`` `*`i`*`)` returns `#t`; otherwise it returns `#f`.
**(markdown-list-item? *****obj*****)**
Returns `#t` if *obj* is a variant of algebraic datatype `list-item`.
**(markdown-list-item=? *****lhs rhs*****)**
Returns `#t` if markdown list items *lhs* and *rhs* are equals; otherwise it returns `#f`.
**(markdown-text? *****obj*****)**
Returns `#t` if *obj* is a proper list of objects *o* for which `(markdown-inline?`` `*`o`*`)` returns `#t`; otherwise it returns `#f`.
**(markdown-inline? *****obj*****)**
Returns `#t` if *obj* is a variant of algebraic datatype `inline`.
**(markdown-inline=? *****lhs rhs*****)**
Returns `#t` if markdown inline text *lhs* and *rhs* are equals; otherwise it returns `#f`.
**(markdown? *****obj*****)**
Returns `#t` if *obj* is a valid markdown document, i.e. an instance of the `document` variant of datatype `block`; returns `#f` otherwise.
**(markdown=? *****lhs rhs*****)**
Returns `#t` if markdown documents *lhs* and *rhs* are equals; otherwise it returns `#f`.
**(markdown *****str*****)**
Parses the text in Markdown format in *str* and returns a representation of the abstract syntax using the algebraic datatypes `block`, `list-item`, and `inline`.
**(markdown->html *****md*****)**
Converts a Markdown document *md* into HTML, represented in form of a string. *md* needs to satisfy the *markdown?* predicate.
**(blocks->html *****bs*****)** \
\&#xNAN;**(blocks->html *****bs tight*****)**
Converts a Markdown block or list of blocks *bs* into HTML, represented in form of a string. *tight?* is a boolean and should be set to true if the conversion should consider tight typesetting (see CommonMark specification for details).
**(text->html *****txt*****)**
Converts Markdown inline text or list of inline texts *txt* into HTML, represented in form of a string.
**(markdown->html-doc *****md*****)** \
\&#xNAN;**(markdown->html-doc *****md style*****)**\
\&#xNAN;**(markdown->html-doc *****md style codestyle*****)**\
\&#xNAN;**(markdown->html-doc *****md style codestyle cblockstyle*****)**\
\&#xNAN;**(markdown->html-doc *****md style codestyle cblockstyle colors*****)**
Converts a Markdown document *md* into a styled HTML document, represented in form of a string. *md* needs to satisfy the *markdown?* predicate. *style* is a list with up to three elements: *(size font color)*. It specifies the default text style of the document. *size* is the point size of the font, *font* is a font name, and *color* is a HTML color specification (e.g. `"#FF6789"`). *codestyle* specifies the style of inline code in the same format. *colors* is a list of HTML color specifications for the following document elements in this order: the border color of code blocks, the color of blockquote "bars", the color of H1, H2, H3 and H4 headers.
**(markdown->sxml *****md*****)**
Converts a Markdown document *md* into SXML representation. *md* needs to satisfy the *markdown?* predicate.
**(blocks->sxml *****bs*****)** \
\&#xNAN;**(blocks->sxml *****bs tight*****)**
Converts a Markdown block or list of blocks *bs* into SXML representation. *tight?* is a boolean and should be set to true if the conversion should consider tight typesetting (see CommonMark specification for details).
**(text->sxml *****txt*****)**
Converts Markdown inline text or a list of inline text objects *txt* into SXML representation.
**(markdown->raw-string *****md*****)**
Converts a Markdown document *md* into raw text, a string ignoring any markup. *md* needs to satisfy the *markdown?* predicate.
**(blocks->raw-string *****bs*****)**
Converts a Markdown block or list of blocks *bs* into raw text, a string ignoring any markup.
**(text->raw-string *****text*****)**
Converts given inline text *text* into raw text, a string representation ignoring any markup in *text*. *text* needs to satisfy the *markdown-text?* predicate.
**(text->string *****text*****)**
Converts given inline text *text* into a string representation which encodes markup in *text* using Markdown syntax. *text* needs to satisfy the *markdown-text?* predicate.
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