doc/ref/spec.md: introduce colons as separators
- remove quoted identifiers in favor of using strings
and better aliasing
Issue #60
Change-Id: I6c6840f59e89678bced077a6cbd9bbe87ec4086e
Reviewed-on: https://cue-review.googlesource.com/c/cue/+/3551
Reviewed-by: Marcel van Lohuizen <mpvl@golang.org>
diff --git a/doc/ref/spec.md b/doc/ref/spec.md
index 5e83c51..a748a57 100644
--- a/doc/ref/spec.md
+++ b/doc/ref/spec.md
@@ -177,16 +177,9 @@
### Identifiers
Identifiers name entities such as fields and aliases.
-Identifier may be simple or quoted.
-A simple identifier is a sequence of one or more letters (which includes `_`) and digits.
+An identifier is a sequence of one or more letters (which includes `_`) and digits.
It may not be `_`.
The first character in an identifier must be a letter.
-Any sequence of letters, digits or `-` enclosed in
-backticks "`" make an identifier.
-The backticks are not part of the identifier.
-This allows one to refer to fields that are labeled
-with keywords or other identifiers that would
-otherwise not be legal.
<!--
TODO: allow identifiers as defined in Unicode UAX #31
@@ -196,11 +189,8 @@
-->
```
-identifier = simple_identifier | quoted_identifier .
-simple_identifier = letter { letter | unicode_digit } .
-quoted_identifier = "`" { letter | unicode_digit | "-" } "`" .
+identifier = letter { letter | unicode_digit } .
```
-<!-- TODO: relax to allow other punctuation -->
```
a
@@ -327,7 +317,7 @@
float_lit = decimals "." [ decimals ] [ exponent ] |
decimals exponent |
"." decimals [ exponent ].
-exponent = ( "e" | "E" ) [ "+" | "-" ] decimals .
+exponent = ( "e" | "E" ) [ "+" | "-" ] decimals .
```
<!--
TODO: consider allowing Exo (and up), if not followed by a sign
@@ -1169,16 +1159,14 @@
-->
```
StructLit = "{" { Declaration "," } [ "..." ] "}" .
-Declaration = FieldDecl | DefinitionDecl | AliasDecl | Comprehension | Embedding .
-FieldDecl = Label { Label } ":" Expression { attribute } .
-DefinitionDecl = Label "::" Expression { attribute } .
+Declaration = Field | Alias | Comprehension | Embedding .
Embedding = Expression .
+Field = Label { Label } Expression .
+Alias = identifier "=" Expression .
-AliasDecl = Label "=" Expression .
-BindLabel = "<" identifier ">" .
-ConcreteLabel = identifier | simple_string_lit .
-ExpressionLabel = BindLabel
-Label = ConcreteLabel [ "?" ] | ExpressionLabel .
+Label = [ identifier "=" ] LabelPath ( ":" | "::" ) .
+LabelPath = LabelExpr [ "?" ] | [ LabelExpr ] Filters .
+LabelExpr = identifier | simple_string_lit .
attribute = "@" identifier "(" attr_elems ")" .
attr_elems = attr_elem { "," attr_elem }
@@ -1878,18 +1866,29 @@
PrimaryExpr =
Operand |
PrimaryExpr Selector |
+ PrimaryExpr Query |
PrimaryExpr Index |
PrimaryExpr Slice |
PrimaryExpr Arguments .
-Selector = "." identifier .
+Selector = "." (identifier | simple_string_lit) .
+Query = "." Filters .
+Filters = Filter { Filter } .
+Filter = "[" [ "?" [ ":" ] ] Expression "]" .
Index = "[" Expression "]" .
-Slice = "[" [ Expression ] ":" [ Expression ] "]"
+Slice = "[" [ Expression ] ":" [ Expression ] [ ":" [Expression] ] "]" .
Argument = Expression .
Arguments = "(" [ ( Argument { "," Argument } ) [ "," ] ] ")" .
```
<!---
Argument = Expression | ( identifer ":" Expression ).
+
+// & expression type
+// string_lit: same as label. Arguments is current node.
+// If selector is applied to list, it performs the operation for each
+// element.
+
+TODO: considering allowing decimal_lit for selectors.
--->
```
@@ -1913,9 +1912,24 @@
x.f
```
-denotes the field `f` of the value `x`.
+denotes the element of a <!--list or -->struct `x` identified by `f`.
+<!--For structs, -->`f` must be an identifier or a string literal identifying
+any definition or regular non-optional field.
The identifier `f` is called the field selector.
+
+<!--
+Allowing strings to be used as field selectors obviates the need for
+backquoted identifiers. Note that some standards use names for structs that
+are not standard identifiers (such "Fn::Foo"). Note that indexing does not
+allow access to identifiers.
+-->
+
+<!--
+For lists, `f` must be an integer and follows the same lookup rules as
+for the index operation.
The type of the selector expression is the type of `f`.
+-->
+
If `x` is a package name, see the section on [qualified identifiers](#qualified-identifiers).
<!--
@@ -1935,20 +1949,23 @@
the selector is applied to each element `x1.f | ... | xn.f`.
-->
-Otherwise, if `x` is not a struct, or if `f` does not exist in `x`,
+Otherwise, if `x` is not a <!--list or -->struct,
+or if `f` does not exist in `x`,
the result of the expression is bottom (an error).
In the latter case the expression is incomplete.
The operand of a selector may be associated with a default.
```
T: {
- x: int
- y: 3
+ x: int
+ y: 3
+ "x-y": 4
}
-a: T.x // int
-b: T.y // 3
-c: T.z // _|_ // field 'z' not found in T
+a: T.x // int
+b: T.y // 3
+c: T.z // _|_ // field 'z' not found in T
+d: T."x-y" // 4
e: {a: 1|*2} | *{a: 3|*4}
f: e.a // 4 (default value)
