no-filter-in-filter validation from C to Go

go/cases-to-do.txt

@@ -2,7 +2,8 @@
 BUG:
 
 rrv ./reg-test/cases/case-c-dsl-context-specific-validation.sh
-* subr and more ... also needs check for filter statement within filter commands
+* also needs check for filter statement within filter commands
+* subr and more
 
 rrv ./reg-test/cases/case-c-dsl-forbind-typedecl.sh
 rrv ./reg-test/cases/case-c-dsl-localvar-typedecl.sh

go/reg-test/cases/case-no-filter-in-filter.sh

@@ -0,0 +1,2 @@
+run_mlr put 'filter NR > 2' $indir/s.dkvp
+mlr_expect_fail filter 'filter NR > 2' $indir/s.dkvp

go/reg-test/expected/case-no-filter-in-filter.sh.out

@@ -0,0 +1,7 @@
+
+mlr put filter NR > 2 ./reg-test/input/s.dkvp
+a=wye,b=wye,i=3,x=0.20460330576630303,y=0.33831852551664776
+a=eks,b=wye,i=4,x=0.38139939387114097,y=0.13418874328430463
+
+mlr filter filter NR > 2 ./reg-test/input/s.dkvp
+

go/src/miller/dsl/cst/validate.go

@@ -39,6 +39,7 @@ func ValidateAST(
 		for _, astChild := range ast.RootNode.Children {
 			return validateASTAux(
 				astChild,
+				isFilter,
 				atTopLevel,
 				inLoop,
 				inBeginOrEnd,
@@ -57,6 +58,7 @@ func ValidateAST(
 // ----------------------------------------------------------------
 func validateASTAux(
 	astNode *dsl.ASTNode,
+	isFilter bool,
 	atTopLevel bool,
 	inLoop bool,
 	inBeginOrEnd bool,
@@ -66,6 +68,7 @@ func validateASTAux(
 	isAssignmentLHS bool,
 	isUnset bool,
 ) error {
+	nextIsFilter := isFilter
 	nextAtTopLevel := false
 	nextInLoop := inLoop
 	nextInBeginOrEnd := inBeginOrEnd
@@ -74,6 +77,16 @@ func validateASTAux(
 	nextIsAssignmentLHS := isAssignmentLHS
 	nextIsUnset := isUnset
 
+	if astNode.Type == dsl.NodeTypeFilterStatement {
+		if isFilter {
+			return errors.New(
+				"Miller: filter expressions must not also contain the \"filter\" keyword.",
+			)
+		}
+		nextIsFilter = true
+		nextInUDS = true
+	}
+
 	// Check: begin/end/func/subr must be at top-level
 	if astNode.Type == dsl.NodeTypeBeginBlock {
 		if !atTopLevel {
@@ -206,10 +219,6 @@ func validateASTAux(
 		}
 	}
 
-	// Check: ENV disallowed at left-hand side of assignment? or nah?
-	// Needs to be supported for system() ...
-	// * TODO
-
 	//  - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
 	// Treewalk
 
@@ -219,6 +228,7 @@ func validateASTAux(
 			nextIsUnset = astNode.Type == dsl.NodeTypeUnset
 			err := validateASTAux(
 				astChild,
+				nextIsFilter,
 				nextAtTopLevel,
 				nextInLoop,
 				nextInBeginOrEnd,