Use records instead of dictionaries

preserves-expressions.md

@@ -3,7 +3,7 @@ title: "P-expressions"
 ---
 
 Tony Garnock-Jones <tonyg@leastfixedpoint.com>  
-October 2023. Version 0.1.2.
+October 2023. Version 0.2.0.
 
 This document defines a grammar called *Preserves Expressions*
 (*P-expressions*, *pexprs*) that includes [ordinary Preserves text
@@ -105,35 +105,27 @@ We write ⌜*p*⌝ for the encoding into Preserves of P-expression *p*.
 | ⌜·⌝ : **P-expression** | ⟶ | **Preserves** |
 
 Aside from `Group`, `Block`, `Comma`, `Semicolon`, `Colons`, `Trailer`,
-and empty `Record`, P-expressions are encoded directly as Preserves
-data.
+and `Record`, P-expressions are encoded directly as Preserves data.
 
 {:.pseudocode.equations}
 | ⌜`[`*p* ...`]`⌝  | = | `[`⌜*p*⌝ ...`]`             |
-| ⌜`<`*p* ...`>`⌝  | = | `<`⌜*p*⌝ ...`>`             |
 | ⌜`#{`*p* ...`}`⌝ | = | `#{`⌜*p*⌝ ...`}`            |
 | ⌜`#!`*p*⌝        | = | `#!`⌜*p*⌝                   |
 | ⌜`@`*p* *q*⌝     | = | `@`⌜*p*⌝ ⌜*q*⌝              |
 | ⌜*p*⌝            | = | *p* when *p* ∈ **Atom** |
 
-Everything else is encoded as Preserves
-dictionaries.[^encoding-rationale]
-
-[^encoding-rationale]: In principle, it would be nice to use *records*
-    for this purpose, but if we did so we would have to also encode
-    usages of records!
+Everything else is encoded as Preserves records.
 
 {:.pseudocode.equations}
-| ⌜`<>`⌝          | = | `{r:[]}`            |
-| ⌜`(`*p* ...`)`⌝ | = | `{g:[`⌜*p*⌝ ...`]}` |
-| ⌜`{`*p* ...`}`⌝ | = | `{b:[`⌜*p*⌝ ...`]}` |
-| ⌜`,`⌝           | = | `{s:|,|}`           |
-| ⌜`;`⌝           | = | `{s:|;|}`           |
-| ⌜`:` ...⌝       | = | `{s:|:` ...`|}`     |
-| ⌜*t*⌝           | = | ⌜*a*⌝ ... `{}`, where *a* ... are the annotations in *t* and *t* ∈ **Trailer** |
+| ⌜`<`*p* ...`>`⌝ | = | `<r` ⌜*p*⌝ ...`>` |
+| ⌜`(`*p* ...`)`⌝ | = | `<g` ⌜*p*⌝ ...`>` |
+| ⌜`{`*p* ...`}`⌝ | = | `<b` ⌜*p*⌝ ...`>` |
+| ⌜`,`⌝           | = | `<s |,|>`         |
+| ⌜`;`⌝           | = | `<s |;|>`         |
+| ⌜`:` ...⌝       | = | `<s |:` ...`|>`   |
+| ⌜*t*⌝           | = | ⌜*a*⌝ ... `<a>`, where *a* ... are the annotations in *t* and *t* ∈ **Trailer** |
 
-The empty dictionary `{}` acts as an anchor for the annotations in a
-`Trailer`.
+The record `<a>` acts as an anchor for the annotations in a `Trailer`.
 
 We overload the ⌜·⌝ notation for encoding whole `Document`s into
 sequences of Preserves values.
@@ -179,25 +171,25 @@ text-syntax encodings.
 
 ```preserves
  ⌜<date 1821 (lookup-month "February") 3>⌝
-= <date 1821 {g:[lookup-month "February"]} 3>
+= <r date 1821 <g lookup-month "February"> 3>
 ```
 
 ```preserves
  ⌜<>⌝
-= {r:[]}
+= <r>
 ```
 
 ```preserves
  ⌜(begin (println! (+ 1 2)) (+ 3 4))⌝
-= {g:[begin {g:[println! {g:[+ 1 2]}]} {g:[+ 3 4]}]}
+= <g begin <g println! <g + 1 2>> <g + 3 4>>
 ```
 
 ```preserves
  ⌜()⌝
-= {g:[]}
+= <g>
 
  ⌜[() () ()]⌝
-= [{g:[]}, {g:[]}, {g:[]}]
+= [<g>, <g>, <g>]
 ```
 
 ```preserves
@@ -209,20 +201,20 @@ text-syntax encodings.
       }
       tearDown();
   }⌝
-= {b:[
-      setUp {g:[]} {s:|;|}
+= <b
+      setUp <g> <s |;|>
       # Now enter the loop
-      loop {s:|:|} {b:[
-          greet {g:["World"]} {s:|;|}
-      ]}
-      tearDown {g:[]} {s:|;|}
-  ]}
+      loop <s |:|> <b
+          greet <g "World"> <s |;|>
+      >
+      tearDown <g> <s |;|>
+  >
 ```
 
 ```preserves
  ⌜[1 + 2.0, print "Hello", predicate: #t, foo, #!remote, bar]⌝
-= [1 + 2.0 {s:|,|} print "Hello" {s:|,|} predicate {s:|:|} #t {s:|,|}
-   foo {s:|,|} #!remote {s:|,|} bar]
+= [1 + 2.0 <s |,|> print "Hello" <s |,|> predicate <s |:|> #t <s |,|>
+   foo <s |,|> #!remote <s |,|> bar]
 ```
 
 ```preserves
@@ -230,10 +222,10 @@ text-syntax encodings.
       optional name: string,
       address: Address,
   }⌝
-= {b:[
-      optional name {s:|:|} string {s:|,|}
-      address {s:|:|} Address {s:|,|}
-  ]}
+= <b
+      optional name <s |:|> string <s |,|>
+      address <s |:|> Address <s |,|>
+  >
 ```
 
 ### Whole `Document`s
@@ -244,12 +236,12 @@ text-syntax encodings.
       # example of a comment at the end of a dictionary
   }
   # example of a comment at the end of the input file⌝
-= [ {b:[
-        key {s:|:|} value
-        @"example of a comment at the end of a dictionary" {}
-    ]}
+= [ <b
+        key <s |:|> value
+        @"example of a comment at the end of a dictionary" <a>
+    >
     @"example of a comment at the end of the input file"
-    {}
+    <a>
   ]
 ```