lang: ast: try to not use sub-graphs if possible

lang/ast/structs.go

@@ -8191,6 +8191,39 @@ func (obj *ExprCall) Graph(env map[string]interfaces.Func) (*pgraph.Graph, inter
 		return nil, nil, errwrap.Wrapf(err, "could not get the type of the function")
 	}
 
+	// Loop over the arguments, add them to the graph, but do _not_ connect them
+	// to the function vertex. Instead, each time we need to generate a sub-graph
+	// (perhaps only once if the function expression is constant), the FuncValue
+	// (either that constant expression or a FuncValue which CallFunc receives
+	// from upstream) creates the corresponding subgraph and connects these
+	// arguments to it.
+	var argFuncs []interfaces.Func
+	for i, arg := range obj.Args {
+		argGraph, argFunc, err := arg.Graph(env)
+		if err != nil {
+			return nil, nil, errwrap.Wrapf(err, "could not get graph for arg %d", i)
+		}
+		graph.AddGraph(argGraph)
+		argFuncs = append(argFuncs, argFunc)
+	}
+
+	// if obj.expr is a CallableExpr and that expression is constant, then we
+	// don't need CallFunc because the same sub-graph will be used for the
+	// entirety of the execution. Instead, embed that sub-graph in the main graph.
+	staticValue, err := obj.expr.Value()
+	if staticValue != nil {
+		if fullFuncValue, isFuncValue := staticValue.(*full.FuncValue); isFuncValue {
+			// The function is constant, so we can embed the sub-graph directly.
+			fakeTxn := panic("TODO: create a txn which adds nodes to the graph")
+			outputFunc, err := fullFuncValue.Call(fakeTxn, argFuncs)
+			if err != nil {
+				return nil, nil, errwrap.Wrapf(err, "could not call the function")
+			}
+			graph.AddGraph(fakeTxn.Graph())
+			return graph, outputFunc, nil
+		}
+	}
+
 	// Find the vertex which produces the FuncValue.
 	var funcValueFunc interfaces.Func
 	if _, isParam := obj.expr.(*ExprParam); isParam {
@@ -8215,20 +8248,6 @@ func (obj *ExprCall) Graph(env map[string]interfaces.Func) (*pgraph.Graph, inter
 		funcValueFunc = topLevelFunc
 	}
 
-	// Loop over the arguments, add them to the graph, but do _not_ connect them
-	// to the function vertex. Instead, each time the call vertex (which we
-	// create below) receives a FuncValue from the function node, it creates the
-	// corresponding subgraph and connects these arguments to it.
-	var argFuncs []interfaces.Func
-	for i, arg := range obj.Args {
-		argGraph, argFunc, err := arg.Graph(env)
-		if err != nil {
-			return nil, nil, errwrap.Wrapf(err, "could not get graph for arg %d", i)
-		}
-		graph.AddGraph(argGraph)
-		argFuncs = append(argFuncs, argFunc)
-	}
-
 	// Add a vertex for the call itself.
 	edgeName := structs.CallFuncArgNameFunction
 	callFunc := &structs.CallFunc{