4166e68 · urbit.org · git.iko.soy

321surrounded on both sides by text that matches a pair of `rule`s, which is discarded.

322

323```

324−> (scan "(42)" (ifix [lit rit] (jest '42')))

324+> (scan "(42)" (ifix [pal par] (jest '42')))

325'42'

326```

327−`+lit` and `+rit` are shorthand for `(just '(')` and `(just ')')`, respectively. All

327+`+pal` and `+par` are shorthand for `(just '(')` and `(just ')')`, respectively. All

328ASCII glyphs have counterparts of this sort, documented

329[here](@/docs/reference/library/4h.md).

330

520:: expr-parse: parse arithmetic expressions

521::

522|= xprs=tape

523−|^ (scan xprs tape)

523+|^ (scan xprs expr)

524++ factor

525 %+ knee *@ud

526 |. ~+

527 ;~ pose

528 dem

529− (ifix [lit rit] expr)

529+ (ifix [pal par] expr)

530 ==

531++ turm

532 %+ knee *@ud

563 |. ~+

564 ;~ pose

565 dem

566− (ifix [lit rit] expr)

566+ (ifix [pal par] expr)

567 ==

568```

569

570A _factor_ is either a decimal number or an expression surrounded by parentheses. Put

571into Hoon terms, a decimal number is parsed by the `rule` `+dem` and an

572expression is parsed by removing the surrounding parentheses and then passing

573−the result to the expression parser arm `+expr`, given by the `rule` `(ifix [lit

573+the result to the expression parser arm `+expr`, given by the `rule` `(ifix [pal

574−rit] expr)`. Since we want to parse our expression with one or the other, we

574+par] expr)`. Since we want to parse our expression with one or the other, we

575chain these two `rule`s together using the monadic applicator rune `;~` along

576with `+pose`, which says to try each rule in succession until one of them works.

577

584 |. ~+

585 ;~ pose

586 dem

587− (ifix [lit rit] expr)

587+ (ifix [pal par] expr)

588 ==

589```

590`~+` is used to cache the parser, so that it does not need to be computed over