35 2D Syntax
The
unstable/2d language installs
#2d reader support in the readtable, and then chains to the reader of
another language that is specified immediately after
unstable/2d.
The #2d syntax extension adds the ability use
a two-dimensional grid syntax. That is, you can drawn an ASCII-art
grid and then treat that as an expression. For example,
here is a simple equality function that operates on pairs and
numbers, written using a #2d conditional expression:
| #lang unstable/2d racket |
| |
| (define (same? a b) |
| #2dcond |
| ╔═════════════╦═══════════════════════╦═════════════╗ |
| ║ ║ (pair? a) ║ (number? a) ║ |
| ╠═════════════╬═══════════════════════╬═════════════╣ |
| ║ (pair? b) ║ (and (same? (car a) ║ #f ║ |
| ║ ║ (car b)) ║ ║ |
| ║ ║ (same? (cdr a) ║ ║ |
| ║ ║ (cdr b))) ║ ║ |
| ╠═════════════╬═══════════════════════╬═════════════╣ |
| ║ (number? b) ║ #f ║ (= a b) ║ |
| ╚═════════════╩═══════════════════════╩═════════════╝) |
This notation works in two stages: reading, and parsing (just as in
Racket in general). The reading stage converts anything that begins
with #2d into a parenthesized expression (possibly signaling
errors if the ═ and ║ and ╬
characters do not line up in the right places).
Since the first line contains #2dcond, the reader will
produce a sequence whose first position is the identifier 2dcond.
That macro will take over and then expand into ordinary conditional
expressions, in this case figuring out whether or not the inputs
are pairs or numbers and evaluating the code in the appropriate cell.
At the reader level, the syntax #2d notation checks
the number of columns in the first row and uses that as a guide
for where subsequent rows may appear. Once that first row is set,
it serves as a guide to where the columns may appear in subsequent
rows, although following columns may be merged.
This merging can simplify
some uses of #2d expressions. For example, consider this
expression that captures subtyping relationships between a few of the
Typed Racket numeric types, this time using a #2d match
expression:
| #lang unstable/2d racket |
| |
| (define (subtype? a b) |
| #2dmatch |
| ╔══════════╦══════════╦═══════╦══════════╗ |
| ║ a b ║ 'Integer ║ 'Real ║ 'Complex ║ |
| ╠══════════╬══════════╩═══════╩══════════╣ |
| ║ 'Integer ║ #t ║ |
| ╠══════════╬══════════╗ ║ |
| ║ 'Real ║ ║ ║ |
| ╠══════════╣ ╚═══════╗ ║ |
| ║ 'Complex ║ #f ║ ║ |
| ╚══════════╩══════════════════╩══════════╝) |
There are a number of cell walls missing here, but this is still a
well-formed #2d expression. In this case, the 2dmatch
treats any of the situations that fall into the larger regions as
the same.
35.1 2D Cond
(2dcond cond-content)
|
| |
| cond-content | | = | | | | | | | | | | question-row | | = | | empty-cell question-cell ⋯ | | | | | | | | body-row | | = | | question-cell exprs-cell ⋯ | | | | | | | | question-cell | | = | | | ╔═════════════╗ | | ║question-expr║ | | ╚═════════════╝ |
| | | | | | | | empty-cell | | = | | | | | | | | | | exprs-cell | | = | | | ╔═════════════╗ | | ║expr expr ...║ | | ╚═════════════╝ |
|
|
Evaluates the first row of question expressions until
one of them returns a true value (signaling an error if none do),
then evaluates the first column of question expressions until
one of them returns a true value (signaling an error if none do),
and then evaluates the cell in the middle where both point to,
returning the result of the last expression in that cell.
35.2 2D Match
(2dmatch match-content)
|
| |
| match-content | | = | | | match-first-row | | match-row | | ⋮ |
| | | | | | | | match-first-row | | = | | two-expr-cell match-pat-cell ⋯ | | | | | | | | match-row | | = | | match-pat-cell exprs-cell ⋯ | | | | | | | | two-expr-cell | | = | | | ╔═════════════════╗ | | ║col-expr row-expr║ | | ╚═════════════════╝ |
| | | | | | | | match-pat-cell | | = | | | | | | | | | | exprs-cell | | = | | | ╔═════════════╗ | | ║expr expr ...║ | | ╚═════════════╝ |
|
|
Matches col-expr against each of patterns
in the first column of the table and matches row-expr
against each of the patterns in the row row, and then evaluates
the corresponding exprs-cell, returning the value of the
last expression in that cell.
35.3 2D Tabular
(2dmatch tabular-content)
|
| |
| tabular-content | | = | | | | | | | | | | | | | | | | | tabular-row | | = | | tabular-cell ⋯ | | | | | | | | tabular-cell | | = | | | ╔════════════════╗ | | ║tabular-expr ...║ | | ╚════════════════╝ |
| | | | | | | | style-cell | | = | | | ╔═════════════════╗ | | ║style-content ...║ | | ╚═════════════════╝ |
| | | | | | | | style-content | | = | | #:style style-expr | | | | | | | #:sep sep-expr | | | | | | | #:ignore-first-row |
|
| |
| | sep-expr | | : | | (or/c block? content? #f) |
| | tabular-expr | | : | | (or/c block? content?) |
|
Constructs a tabular matching the given cells.
If a cell spans multiple columns, then the resulting
tabular has 'cont in the corresponding
list element. No cells may span rows.
The #:style and #:sep arguments are just passed
to tabular.
If the #:ignore-first-row keyword is provided, then the first
row of the 2dtabular expression is ignored. This can be used
in case the first row of the rendered table should not have all of the
columns (as #2d syntax requires that the first row contain
a cell for each column that appears in the table).