Empty records

preserves-expressions.md

@@ -3,7 +3,7 @@ title: "P-expressions"
 ---
 
 Tony Garnock-Jones <tonyg@leastfixedpoint.com>  
-October 2023. Version 0.1.1.
+October 2023. Version 0.1.2.
 
 This document defines a grammar called *Preserves Expressions*
 (*P-expressions*, *pexprs*) that includes [ordinary Preserves text
@@ -53,6 +53,10 @@ with `Block` everywhere it is mentioned.
 
              Block = "{" *Value ws "}"
 
+Syntax for `Record` is loosened to allow empty angle brackets.
+
+            Record = "<" *Value ws ">"
+
 New syntax for explicit uninterpreted grouping of sequences of values is
 introduced, and added to class `Value`.
 
@@ -86,7 +90,7 @@ While the ordinary text syntax forbids comments in these positions,
 P-expressions allow them:
 
          Document =/ *Value Trailer ws
-           Record =/ "<" Value *Value Trailer ws ">"
+           Record =/  "<" *Value Trailer ws ">"
          Sequence =/  "[" *Value Trailer ws "]"
               Set =/ "#{" *Value Trailer ws "}"
             Block =/  "{" *Value Trailer ws "}"
@@ -101,8 +105,8 @@ We write ⌜*p*⌝ for the encoding into Preserves of P-expression *p*.
 | ⌜·⌝ : **P-expression** | ⟶ | **Preserves** |
 
 Aside from the special classes `Group`, `Block`, `Comma`, `Semicolon`,
-`Colons`, or `Trailer`, P-expressions are encoded directly as Preserves
-data.
+`Colons`, `Trailer`, or empty `Record`, P-expressions are encoded
+directly as Preserves data.
 
 {:.pseudocode.equations}
 | ⌜`[`*p* ...`]`⌝  | = | `[`⌜*p*⌝ ...`]`             |
@@ -112,7 +116,7 @@ data.
 | ⌜`@`*p* *q*⌝     | = | `@`⌜*p*⌝ ⌜*q*⌝              |
 | ⌜*p*⌝            | = | *p* when *p* ∈ **Atom** |
 
-All members of the special classes are encoded as Preserves
+Everything else is encoded as Preserves
 dictionaries[^encoding-rationale].
 
 [^encoding-rationale]: In principle, it would be nice to use *records*
@@ -120,6 +124,7 @@ dictionaries[^encoding-rationale].
     usages of records!
 
 {:.pseudocode.equations}
+| ⌜`<>`⌝          | = | `{r:[]}`            |
 | ⌜`(`*p* ...`)`⌝ | = | `{g:[`⌜*p*⌝ ...`]}` |
 | ⌜`{`*p* ...`}`⌝ | = | `{b:[`⌜*p*⌝ ...`]}` |
 | ⌜`,`⌝           | = | `{s:|,|}`           |
@@ -153,8 +158,9 @@ classes mentioned above.
  1. Every `Group` or `Semicolon` that appears is an error.
  2. Every `Colons` with two or more colons in it is an error.
  3. Every `Comma` that appears is discarded.
- 3. Every `Trailer` that appears is an error.[^discard-trailers-instead-of-error]
- 4. Every `Block` must contain triplets of `Value`, `Colons` (with a
+ 4. Every `Trailer` that appears is an error.[^discard-trailers-instead-of-error]
+ 5. Every `Record` with no values in it is an error.
+ 6. Every `Block` must contain triplets of `Value`, `Colons` (with a
     single colon), `Value`. Any `Block` not following this pattern is an
     error. Each `Block` following the pattern is translated to a
     `Dictionary` containing a key/value pair for each triplet.
@@ -176,6 +182,11 @@ text-syntax encodings.
 = <date 1821 {g:[lookup-month "February"]} 3>
 ```
 
+```preserves
+ ⌜<>⌝
+= {r:[]}
+```
+
 ```preserves
  ⌜(begin (println! (+ 1 2)) (+ 3 4))⌝
 = {g:[begin {g:[println! {g:[+ 1 2]}]} {g:[+ 3 4]}]}