patch 8.2.4526: Vim9: cannot set variables to a null value

Problem:    Vim9: cannot set variables to a null value.
Solution:   Add null_list, null_job, etc.

runtime/doc/vim9.txt

@@ -94,8 +94,20 @@ script and `:def` functions; details are below:
 	def CallMe(count: number, message: string): bool
 - Call functions without `:call`: >
 	writefile(['done'], 'file.txt')
-- You cannot use old Ex commands `:xit`, `:t`, `:k`, `:append`, `:change`,
-  `:insert`, `:open`, and `:s` or `:d` with only flags.
+- You cannot use old Ex commands:
+	`:Print`
+	`:append`
+	`:change`
+	`:d`  directly followed by 'd' or 'p'.
+	`:insert`
+	`:k`
+	`:mode`
+	`:open`
+	`:s`  with only flags
+	`:t`
+  	`:xit`
+- Some commands, especially those used for flow control, cannot be shortened.
+  E.g., `:throw` cannot be written as `:th`. *E839*
 - You cannot use curly-braces names.
 - A range before a command must be prefixed with a colon: >
 	:%s/this/that
@@ -305,7 +317,7 @@ function, the function does not need to be defined more than once: >
 
 
 Variable declarations with :var, :final and :const ~
-				*vim9-declaration* *:var*
+				*vim9-declaration* *:var* *E1079*
 				*E1017* *E1020* *E1054* *E1087* *E1108* *E1124*
 Local variables need to be declared with `:var`.  Local constants need to be
 declared with `:final` or `:const`.  We refer to both as "variables" in this
@@ -375,6 +387,9 @@ In Vim9 script `:let` cannot be used.  An existing variable is assigned to
 without any command.  The same for global, window, tab, buffer and Vim
 variables, because they are not really declared.  Those can also be deleted
 with `:unlet`.
+							*E1065*
+You cannot use `:va` to declare a variable, it must be written with the full
+name `:var`.  Just to make sure it is easy to read.
 							*E1178*
 `:lockvar` does not work on local variables.  Use `:const` and `:final`
 instead.
@@ -952,10 +967,37 @@ always converted to string: >
 Simple types are Number, Float, Special and Bool.  For other types |string()|
 should be used.
 						*false* *true* *null* *E1034*
-In Vim9 script one can use "true" for v:true, "false" for v:false and "null"
-for v:null.  When converting a boolean to a string "false" and "true" are
-used, not "v:false" and "v:true" like in legacy script.  "v:none" is not
-changed, it is only used in JSON and has no equivalent in other languages.
+In Vim9 script one can use the following predefined values: >
+	true
+	false
+	null
+	null_blob
+	null_channel
+	null_dict
+	null_function
+	null_job
+	null_list
+	null_partial
+	null_string
+`true` is the same as `v:true`, `false` the same as `v:false`, `null` the same
+as `v:null`.
+
+While `null` has the type "special", the other "null_" types have the type
+indicated by their name.  Quite often a null value is handled the same as an
+empty value, but not always.  The values can be useful to clear a script-local
+variable, since they cannot be deleted with `:unlet`.  E.g.: >
+	var theJob = job_start(...)
+	# let the job do its work
+	theJob = null_job
+
+The values can also be useful as the default value for an argument: >
+	def MyFunc(b: blob = null_blob)
+	   if b == null_blob
+	      # b argument was not given
+
+When converting a boolean to a string `false` and `true` are used, not
+`v:false` and `v:true` like in legacy script.  `v:none` has no `none`
+replacement, it has no equivalent in other languages.
 
 Indexing a string with [idx] or taking a slice with [idx : idx] uses character
 indexes instead of byte indexes.  Composing characters are included.

src/eval.c

@@ -943,6 +943,7 @@ get_lval(
 		    type_list = &SCRIPT_ITEM(current_sctx.sc_sid)->sn_type_list;
 		else
 		{
+		    // TODO: should we give an error here?
 		    type_list = &tmp_type_list;
 		    ga_init2(type_list, sizeof(type_T), 10);
 		}
@@ -3482,6 +3483,100 @@ eval_leader(char_u **arg, int vim9)
     return OK;
 }
 
+/*
+ * Check for a predefined value "true", "false" and "null.*".
+ * Return OK when recognized.
+ */
+    int
+handle_predefined(char_u *s, int len, typval_T *rettv)
+{
+    switch (len)
+    {
+	case 4: if (STRNCMP(s, "true", 4) == 0)
+		{
+		    rettv->v_type = VAR_BOOL;
+		    rettv->vval.v_number = VVAL_TRUE;
+		    return OK;
+		}
+		if (STRNCMP(s, "null", 4) == 0)
+		{
+		    rettv->v_type = VAR_SPECIAL;
+		    rettv->vval.v_number = VVAL_NULL;
+		    return OK;
+		}
+		break;
+	case 5: if (STRNCMP(s, "false", 5) == 0)
+		{
+		    rettv->v_type = VAR_BOOL;
+		    rettv->vval.v_number = VVAL_FALSE;
+		    return OK;
+		}
+		break;
+#ifdef FEAT_JOB_CHANNEL
+	case 8: if (STRNCMP(s, "null_job", 8) == 0)
+		{
+		    rettv->v_type = VAR_JOB;
+		    rettv->vval.v_job = NULL;
+		    return OK;
+		}
+		break;
+#endif
+	case 9:
+		if (STRNCMP(s, "null_", 5) != 0)
+		    break;
+		if (STRNCMP(s + 5, "list", 4) == 0)
+		{
+		    rettv->v_type = VAR_LIST;
+		    rettv->vval.v_list = NULL;
+		    return OK;
+		}
+		if (STRNCMP(s + 5, "dict", 4) == 0)
+		{
+		    rettv->v_type = VAR_DICT;
+		    rettv->vval.v_dict = NULL;
+		    return OK;
+		}
+		if (STRNCMP(s + 5, "blob", 4) == 0)
+		{
+		    rettv->v_type = VAR_BLOB;
+		    rettv->vval.v_blob = NULL;
+		    return OK;
+		}
+		break;
+	case 11: if (STRNCMP(s, "null_string", 11) == 0)
+		{
+		    rettv->v_type = VAR_STRING;
+		    rettv->vval.v_string = NULL;
+		    return OK;
+		}
+		break;
+	case 12:
+#ifdef FEAT_JOB_CHANNEL
+		if (STRNCMP(s, "null_channel", 12) == 0)
+		{
+		    rettv->v_type = VAR_CHANNEL;
+		    rettv->vval.v_channel = NULL;
+		    return OK;
+		}
+#endif
+		if (STRNCMP(s, "null_partial", 12) == 0)
+		{
+		    rettv->v_type = VAR_PARTIAL;
+		    rettv->vval.v_partial = NULL;
+		    return OK;
+		}
+		break;
+	case 13: if (STRNCMP(s, "null_function", 13) == 0)
+		{
+		    rettv->v_type = VAR_FUNC;
+		    rettv->vval.v_string = NULL;
+		    return OK;
+		}
+		break;
+    }
+    return FAIL;
+}
+
 /*
  * Handle sixth level expression:
  *  number		number constant
@@ -3757,26 +3852,11 @@ eval7(
 		ret = FAIL;
 	    else if (evaluate)
 	    {
-		// get the value of "true", "false" or a variable
-		if (len == 4 && vim9script && STRNCMP(s, "true", 4) == 0)
-		{
-		    rettv->v_type = VAR_BOOL;
-		    rettv->vval.v_number = VVAL_TRUE;
-		    ret = OK;
-		}
-		else if (len == 5 && vim9script && STRNCMP(s, "false", 5) == 0)
-		{
-		    rettv->v_type = VAR_BOOL;
-		    rettv->vval.v_number = VVAL_FALSE;
-		    ret = OK;
-		}
-		else if (len == 4 && vim9script && STRNCMP(s, "null", 4) == 0)
-		{
-		    rettv->v_type = VAR_SPECIAL;
-		    rettv->vval.v_number = VVAL_NULL;
-		    ret = OK;
-		}
-		else
+		// get the value of "true", "false", etc. or a variable
+		ret = FAIL;
+		if (vim9script)
+		    ret = handle_predefined(s, len, rettv);
+		if (ret == FAIL)
 		{
 		    name_start = s;
 		    ret = eval_variable(s, len, 0, rettv, NULL,

src/evalvars.c

@@ -999,6 +999,11 @@ ex_let_vars(
     listitem_T	*item;
     typval_T	ltv;
 
+    if (tv->v_type == VAR_VOID)
+    {
+	emsg(_(e_cannot_use_void_value));
+	return FAIL;
+    }
     if (*arg != '[')
     {
 	// ":let var = expr" or ":for var in list"

src/proto/eval.pro

@@ -42,6 +42,7 @@ int eval1(char_u **arg, typval_T *rettv, evalarg_T *evalarg);
 void eval_addblob(typval_T *tv1, typval_T *tv2);
 int eval_addlist(typval_T *tv1, typval_T *tv2);
 int eval_leader(char_u **arg, int vim9);
+int handle_predefined(char_u *s, int len, typval_T *rettv);
 int check_can_index(typval_T *rettv, int evaluate, int verbose);
 void f_slice(typval_T *argvars, typval_T *rettv);
 int eval_index_inner(typval_T *rettv, int is_range, typval_T *var1, typval_T *var2, int exclusive, char_u *key, int keylen, int verbose);

src/testdir/test_expr.vim

@@ -157,12 +157,28 @@ endfunc
 
 func Test_loop_over_null_list()
   let lines =<< trim END
-      VAR null_list = test_null_list()
-      for i in null_list
+      VAR nulllist = test_null_list()
+      for i in nulllist
         call assert_report('should not get here')
       endfor
   END
   call v9.CheckLegacyAndVim9Success(lines)
+
+  let lines =<< trim END
+      var nulllist = null_list
+      for i in nulllist
+        call assert_report('should not get here')
+      endfor
+  END
+  call v9.CheckDefAndScriptSuccess(lines)
+
+  let lines =<< trim END
+      let nulllist = null_list
+      for i in nulllist
+        call assert_report('should not get here')
+      endfor
+  END
+  call v9.CheckScriptFailure(lines, 'E121: Undefined variable: null_list')
 endfunc
 
 func Test_setreg_null_list()

src/testdir/test_vim9_assign.vim

@@ -306,12 +306,44 @@ def Test_assign_register()
 enddef
 
 def Test_reserved_name()
-  for name in ['true', 'false', 'null']
+  var more_names = ['null_job', 'null_channel']
+  if !has('job')
+    more_names = []
+  endif
+
+  for name in ['true',
+               'false',
+               'null',
+               'null_blob',
+               'null_dict',
+               'null_function',
+               'null_list',
+               'null_partial',
+               'null_string',
+               ] + more_names
     v9.CheckDefExecAndScriptFailure(['var ' .. name .. ' =  0'], 'E1034:')
     v9.CheckDefExecAndScriptFailure(['var ' .. name .. ': bool'], 'E1034:')
   endfor
 enddef
 
+def Test_null_values()
+  var lines =<< trim END
+      var b: blob = null_blob
+      var dn: dict<number> = null_dict
+      var ds: dict<string> = null_dict
+      var ln: list<number> = null_list
+      var ls: list<string> = null_list
+      var Ff: func(string): string = null_function
+      var Fp: func(number): number = null_partial
+      var s: string = null_string
+      if has('job')
+        var j: job = null_job
+        var c: channel = null_channel
+      endif
+  END
+  v9.CheckDefAndScriptSuccess(lines)
+enddef
+
 def Test_skipped_assignment()
   var lines =<< trim END
       for x in []

src/testdir/test_vim9_disassemble.vim

@@ -415,6 +415,58 @@ def Test_disassemble_store_member()
         res)
 enddef
 
+if has('job')
+  def s:StoreNull()
+    var ss = null_string
+    var bb = null_blob
+    var dd = null_dict
+    var ll = null_list
+    var Ff = null_function
+    var Pp = null_partial
+    var jj = null_job
+    var cc = null_channel
+  enddef
+
+  def Test_disassemble_assign_null()
+    var res = execute('disass s:StoreNull')
+    assert_match('<SNR>\d*_StoreNull\_s*' ..
+          'var ss = null_string\_s*' ..
+          '\d\+ PUSHS "\[NULL\]"\_s*' ..
+          '\d\+ STORE $\d\_s*' ..
+
+          'var bb = null_blob\_s*' ..
+          '\d\+ PUSHBLOB 0z\_s*' ..
+          '\d\+ STORE $\d\_s*' ..
+
+          'var dd = null_dict\_s*' ..
+          '\d\+ NEWDICT size 0\_s*' ..
+          '\d\+ STORE $\d\_s*' ..
+
+          'var ll = null_list\_s*' ..
+          '\d\+ NEWLIST size 0\_s*' ..
+          '\d\+ STORE $\d\_s*' ..
+
+          'var Ff = null_function\_s*' ..
+          '\d\+ PUSHFUNC "\[none\]"\_s*' ..
+          '\d\+ STORE $\d\_s*' ..
+
+          'var Pp = null_partial\_s*' ..
+          '\d\+ NEWPARTIAL\_s*' ..
+          '\d\+ STORE $\d\_s*' ..
+
+          'var jj = null_job\_s*' ..
+          '\d\+ PUSHJOB "no process"\_s*' ..
+          '\d\+ STORE $\d\_s*' ..
+
+          'var cc = null_channel\_s*' ..
+          '\d\+ PUSHCHANNEL 0\_s*' ..
+          '\d\+ STORE $\d\_s*' ..
+
+          '\d\+ RETURN void',
+          res)
+  enddef
+endif
+
 def s:ScriptFuncStoreIndex()
   var d = {dd: {}}
   d.dd[0] = 0

src/testdir/test_vim9_func.vim

@@ -3326,7 +3326,7 @@ def Test_partial_call()
       var Expr: func(dict<any>): dict<any>
       const Call = Foo(Expr)
   END
-  v9.CheckScriptFailure(lines, 'E1235:')
+  v9.CheckScriptFailure(lines, 'E1031:')
 enddef
 
 def Test_partial_double_nested()

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    4526,
 /**/
     4525,
 /**/