improve error messages if FOR doesn't find proper values to work with

arangod/Aql/AqlValue.cpp

@@ -155,6 +155,35 @@ bool AqlValue::isArray() const noexcept {
   }
 }
 
+std::array<std::string const, 8> AqlValue::typeStrings = {
+  "none",
+  "null",
+  "bool",
+  "number",
+  "string",
+  "object",
+  "array",
+  "unknown"};
+
+std::string const & AqlValue::getTypeString() const noexcept {
+  if(isNone()) {
+    return typeStrings[0];
+  } else if(isNull(true)) {
+     return typeStrings[1];
+  } else if(isBoolean()) {
+     return typeStrings[2];
+  } else if(isNumber()) {
+     return typeStrings[3];
+  } else if(isString()) {
+     return typeStrings[4];
+  } else if(isObject()) {
+     return typeStrings[5];
+  } else if(isArray()){
+     return typeStrings[6];
+  }
+  return typeStrings[7];
+}
+
 /// @brief get the (array) length (note: this treats ranges as arrays, too!)
 size_t AqlValue::length() const {
   switch (type()) {

arangod/Aql/AqlValue.h

@@ -83,6 +83,7 @@ struct AqlValueFromManagedDocument {};
 struct AqlValue final {
  friend struct std::hash<arangodb::aql::AqlValue>;
  friend struct std::equal_to<arangodb::aql::AqlValue>;
+ static std::array<std::string const, 8> typeStrings;
 
  public:
 
@@ -388,6 +389,10 @@ struct AqlValue final {
   /// @brief whether or not the value is an array (note: this treats ranges
   /// as arrays, too!)
   bool isArray() const noexcept;
+
+  // @brief return a string describing the content of this aqlvalue
+  std::string const & getTypeString() const noexcept;
+
   
   /// @brief get the (array) length (note: this treats ranges as arrays, too!)
   size_t length() const;

arangod/Aql/Ast.cpp

@@ -2889,8 +2889,13 @@ AstNode* Ast::optimizeFor(AstNode* node) {
     // right-hand operand to FOR statement is no array
     THROW_ARANGO_EXCEPTION_MESSAGE(
         TRI_ERROR_QUERY_ARRAY_EXPECTED,
+        std::string("collection or ") +
         TRI_errno_string(TRI_ERROR_QUERY_ARRAY_EXPECTED) +
-            std::string(" as operand to FOR loop"));
+        std::string(" as operand to FOR loop; you specified type '") +
+        expression->getValueTypeString() +
+        std::string("' with content '") +
+        expression->toString() +
+        std::string("'"));
   }
 
   // no real optimizations will be done here

arangod/Aql/EnumerateListBlock.cpp

@@ -98,7 +98,7 @@ AqlItemBlock* EnumerateListBlock::getSome(size_t, size_t atMost) {
     AqlValue const& inVarReg = cur->getValueReference(_pos, _inVarRegId);
 
     if (!inVarReg.isArray()) {
-      throwArrayExpectedException();
+      throwArrayExpectedException(inVarReg);
     }
 
     size_t sizeInVar;
@@ -195,7 +195,7 @@ size_t EnumerateListBlock::skipSome(size_t atLeast, size_t atMost) {
     AqlValue const& inVarReg = cur->getValueReference(_pos, _inVarRegId);
     // get the size of the thing we are looping over
     if (!inVarReg.isArray()) {
-      throwArrayExpectedException();
+      throwArrayExpectedException(inVarReg);
     }
     
     size_t sizeInVar;
@@ -253,9 +253,12 @@ AqlValue EnumerateListBlock::getAqlValue(AqlValue const& inVarReg, bool& mustDes
   DEBUG_END_BLOCK();  
 }
 
-void EnumerateListBlock::throwArrayExpectedException() {
+void EnumerateListBlock::throwArrayExpectedException(AqlValue const& value) {
   THROW_ARANGO_EXCEPTION_MESSAGE(
       TRI_ERROR_QUERY_ARRAY_EXPECTED,
+      std::string("collection or ") +
       TRI_errno_string(TRI_ERROR_QUERY_ARRAY_EXPECTED) +
-          std::string(" as operand to FOR loop"));
+          std::string(" as operand to FOR loop; you provided a value of type '") +
+        value.getTypeString () +
+        std::string("'"));
 }

arangod/Aql/EnumerateListBlock.h

@@ -54,7 +54,7 @@ class EnumerateListBlock : public ExecutionBlock {
   AqlValue getAqlValue(AqlValue const&, bool& mustDestroy);
 
   // cppcheck-suppress *
-  void throwArrayExpectedException();
+  void throwArrayExpectedException(AqlValue const& value);
 
  private:
   // current position in the _inVariable