cli: Fix non-string variables and var(; f) variant to delete etc

pkg/interp/funcs.jq

@@ -6,7 +6,7 @@ def help:
   ) | println;
 
 # valid jq identifer, start with alpha or underscore then zero or more alpha, num or underscore
-def _is_ident: test("^[a-zA-Z_][a-zA-Z_0-9]*$");
+def _is_ident: type == "string" and test("^[a-zA-Z_][a-zA-Z_0-9]*$");
 # escape " and \
 def _escape_ident: gsub("(?<g>[\\\\\"])"; "\\\(.g)");
 

pkg/interp/interp.jq

@@ -257,14 +257,14 @@ def inputs:
 def input_filename: _input_filename;
 
 def var: _variables;
-def var($k):
+def var($k; f):
   ( . as $c
-  | if ($k | _is_ident | not) then error("invalid variable name \"\($k)\"") end
-  | _variables(
-      .[$k] |= ($c | if . == null then empty end)
-    )
+  | if ($k | _is_ident | not) then error("invalid variable name: \($k)") end
+  | _variables(.[$k] |= f)
   | empty
   );
+def var($k): . as $c | var($k; $c);
+
 
 # . will have additional array of options taking priority
 # NOTE: is used by go Options()

pkg/interp/testdata/var.fqtest

@@ -17,7 +17,7 @@ null> var
 }
 null> $a
 "aa"
-null> null | var("a")
+null> var("a"; empty)
 null> $a
 error: src:1:0: variable not defined: $a
 null> var
@@ -28,11 +28,13 @@ null> . | repl
 > null> $bb
 "bb"
 > null> ^D
-null> null | var("bb")
+null> var("bb"; empty)
 null> var
 {}
 null> 123 | var("a b")
-error: invalid variable name "a b"
+error: invalid variable name: a b
+null> 123 | var(null)
+error: invalid variable name: null
 null> ^D
 $ fq -d mp3 -i . /test.mp3
 mp3> .frames[0] | var("f")