Ensure {} is a dict, not a set

This fixes the json compatibility issue. Also update some terminology
from "record" to "dict" now that I'm touching things anyway.

docs/syntax.md

@@ -59,11 +59,12 @@ allowed but not required.
       ["Eggplant", "Pepper", "Zuccini"],
     ]
 
-## Records
+## Dictionaries
 
-Records are surrounded by `{}`. Records can be written in json form, where the
-left-hand side is an expression. Then the key and value are separated by `:` and
-the element separator is `,`. A trailing comma is optional.
+Dictionaries, _dicts_ for short, are surrounded by `{}`.  Dicts can be written
+in json form, where the left-hand side is an expression. Then the key and value
+are separated by `:` and the element separator is `,`. A trailing comma is
+optional.
 
     {
       "name": "apple",
@@ -79,7 +80,7 @@ strings precludes serialization to json.
       5 + 5: "X",
     }
 
-Alternatively, records can be written in record form, where the left-hand side
+Alternatively, dicts can be written in record form, where the left-hand side
 is an identifier. Then the key and value are separated by `=` and the element
 separator is `;`. A trailing semicolon is optional. The following value is
 identical to the first one above.
@@ -89,20 +90,25 @@ identical to the first one above.
       flavor = "sweet";
     }
 
-Note, the empty collection `{}` is a set, not a record.
-TODO: It _has_ to be a record, because in json it is a record. Need to fix!
+Note, the empty collection `{}` is a dict, not a set.
 
 ## Sets
 
-Sets are surrounded by `{}` and work otherwise the same as lists. The empty
-collection `{}` is a set, not a record. The following list contains two
-identical sets:
+Sets are surrounded by `{}` and work otherwise the same as lists. The following
+list contains two identical sets:
 
     [
       {"Apple", "Pear"},
       {"Apple", "Pear", "Apple"},
     ]
 
+Note, the empty collection `{}` is a dict, not a set. There is currently no
+literal for the empty set. It is possible to work around this using
+comprehensions:
+
+    // An empty set.
+    {for x in []: x}
+
 ## Let bindings
 
 Values can be bound to names with a let-binding.

golden/error/runtime_collection_kv_scalar.test

@@ -14,4 +14,4 @@ Error: Expected key-value, not a scalar element.
   |
 2 |   "key": "value",
   |        ^
-Note: The collection is a record and not a set, because of this key-value.
+Note: The collection is a dict and not a set, because of this key-value.

golden/error/runtime_collection_list_kv.test

@@ -7,4 +7,4 @@
   |       ^
 Error: Expected scalar element, not key-value.
 
-Help: Key-value pairs are allowed in records, which are enclosed in '{}', not '[]'.
+Help: Key-value pairs are allowed in dicts, which are enclosed in '{}', not '[]'.

golden/error/runtime_collection_scalar_kv.test

@@ -14,4 +14,4 @@ Error: Expected scalar element, not key-value.
   |
 2 |   "single",
   |   ^~~~~~~~
-Note: The collection is a set and not a record, because of this scalar value.
+Note: The collection is a set and not a dict, because of this scalar value.

golden/error/runtime_iter_arity_record.test

@@ -5,10 +5,10 @@
   |
 1 | [for kv in {"key": "value"}: kv]
   |      ^~
-Error: Expected two variables in record iteration.
+Error: Expected two variables in dict iteration.
 
  --> stdin:1:11
   |
 1 | [for kv in {"key": "value"}: kv]
   |            ^~~~~~~~~~~~~~~~
-Note: This is a record, it yields a key and value per iteration.
+Note: This is a dict, it yields a key and value per iteration.

golden/json/empty_dict.test

@@ -0,0 +1,5 @@
+// This is an empty dict, not an empty set.
+{}
+
+# output:
+{}

golden/json/empty_set.test

@@ -1,5 +0,0 @@
-// This is an empty set, not an empty record.
-{}
-
-# output:
-[]

ideas/roadmap.md

@@ -3,14 +3,13 @@
 ## Near term
 
 * [WIP] Colored output for json.
- * Ensure `{}` is a dict, not a set, for json compatibility.
+* [WIP] Update terminology; I am calling them dicts, not records.
  * The ability to turn coloring on or off, respect <https://no-color.org/>.
  * Add assertions.
  * Absorb the `highlight` command into `fmt` and make it use the same coloring?
  * Different output modes, RCL in addition to json (so we can preserve sets);
    possibly yaml output.
  * Preserve insertion order in dicts and sets.
- * Update terminology; I am calling them dicts, not records.
 
 ## Method vs. field access
 

src/eval.rs

@@ -7,7 +7,7 @@
 
 //! Evaluation turns ASTs into values.
 
-use std::collections::{BTreeMap, BTreeSet};
+use std::collections::BTreeMap;
 use std::rc::Rc;
 
 use crate::ast::{BinOp, Expr, FormatFragment, Seq, UnOp};
@@ -23,8 +23,9 @@ pub fn eval(env: &mut Env, expr: &Expr) -> Result<Rc<Value>> {
                 eval_seq(env, seq, &mut out)?;
             }
             match out {
-                // If we have no keys, it’s a set.
-                SeqOut::SetOrDict => Ok(Rc::new(Value::Set(BTreeSet::new()))),
+                // If we have no keys, it’s a dict, because json has no sets,
+                // and `{}` is a json value that should evaluate to itself.
+                SeqOut::SetOrDict => Ok(Rc::new(Value::Map(BTreeMap::new()))),
                 SeqOut::Set(_, values) => {
                     let result = values.into_iter().collect();
                     Ok(Rc::new(Value::Set(result)))