lang: interpolate, funcs: Add a concat function for faster interpolation

This adds a concat function which can be used directly by string
interpolation to avoid having to constantly unify the plus operator
which is much slower at this time.

The new monomorphisms changes caused type unification of a notable
example to go from ~25s to ~5m30s which was obviously not bearable. With
this fix, things are now down to ~6s.

This is an important optimization, but it's also a good reminder that
type unification of polymorphic functions needs to be improved in
general too.

lang/interpolate/interpolate.go

@@ -36,6 +36,10 @@ const (
 	// This can't properly escape a $ in the standard way. It's here in case
 	// someone wants to play with it and examine how the AST stuff worked...
 	UseHilInterpolation = false
+
+	// UseOptimizedConcat uses a simpler to unify concat operator instead of
+	// the normal + operator which uses fancy polymorphic type unification.
+	UseOptimizedConcat = true
 )
 
 // StrInterpolate interpolates a string and returns the representative AST.
@@ -312,6 +316,20 @@ func concatExprListIntoCall(exprs []interfaces.Expr) (interfaces.Expr, error) {
 		return nil, err
 	}
 
+	// Faster variant, but doesn't allow potential future more exotic string
+	// interpolation which would need a more expressive plus operator. I do
+	// not think we'll ever need that, but leave it in for now as a const.
+	if UseOptimizedConcat {
+		return &ast.ExprCall{
+			// NOTE: if we don't set the data field we need Init() called on it!
+			Name: funcs.ConcatFuncName, // concatenate the two strings with concat function
+			Args: []interfaces.Expr{
+				head,    // string arg
+				grouped, // nested function call which returns a string
+			},
+		}, nil
+	}
+
 	return &ast.ExprCall{
 		// NOTE: if we don't set the data field we need Init() called on it!
 		Name: funcs.OperatorFuncName, // concatenate the two strings with + operator

lang/interpolate/interpolate_test.go

@@ -90,11 +90,8 @@ func TestInterpolate0(t *testing.T) {
 	}
 	{
 		fieldName := &ast.ExprCall{
-			Name: funcs.OperatorFuncName,
+			Name: funcs.ConcatFuncName,
 			Args: []interfaces.Expr{
-				&ast.ExprStr{
-					V: "+",
-				},
 				&ast.ExprStr{
 					V: "foo-",
 				},
@@ -365,11 +362,8 @@ func TestInterpolateBasicStmt(t *testing.T) {
 			},
 		}
 		resName := &ast.ExprCall{
-			Name: funcs.OperatorFuncName,
+			Name: funcs.ConcatFuncName,
 			Args: []interfaces.Expr{
-				&ast.ExprStr{
-					V: "+",
-				},
 				&ast.ExprStr{
 					V: "t",
 				},
@@ -421,12 +415,9 @@ func TestInterpolateBasicStmt(t *testing.T) {
 			},
 		}
 		resName := &ast.ExprCall{
-			Name: funcs.OperatorFuncName,
+			Name: funcs.ConcatFuncName,
 			// incorrect sig for this function, and now invalid interpolation
 			Args: []interfaces.Expr{
-				&ast.ExprStr{
-					V: "+",
-				},
 				&ast.ExprStr{
 					V: "t",
 				},
@@ -561,11 +552,8 @@ func TestInterpolateBasicExpr(t *testing.T) {
 			V: "hello ${person_name}",
 		}
 		exp := &ast.ExprCall{
-			Name: funcs.OperatorFuncName,
+			Name: funcs.ConcatFuncName,
 			Args: []interfaces.Expr{
-				&ast.ExprStr{
-					V: "+",
-				},
 				&ast.ExprStr{
 					V: "hello ",
 				},
@@ -611,11 +599,8 @@ func TestInterpolateBasicExpr(t *testing.T) {
 			V: "sweetie${3.14159}", // invalid
 		}
 		exp := &ast.ExprCall{
-			Name: funcs.OperatorFuncName,
+			Name: funcs.ConcatFuncName,
 			Args: []interfaces.Expr{
-				&ast.ExprStr{
-					V: "+",
-				},
 				&ast.ExprStr{
 					V: "sweetie",
 				},
@@ -636,11 +621,8 @@ func TestInterpolateBasicExpr(t *testing.T) {
 			V: "i am: ${sys.hostname()}",
 		}
 		exp := &ast.ExprCall{
-			Name: funcs.OperatorFuncName,
+			Name: funcs.ConcatFuncName,
 			Args: []interfaces.Expr{
-				&ast.ExprStr{
-					V: "+",
-				},
 				&ast.ExprStr{
 					V: "i am: ",
 				},
@@ -662,11 +644,8 @@ func TestInterpolateBasicExpr(t *testing.T) {
 			V: "i am: ${blah(21, 12.3)}",
 		}
 		exp := &ast.ExprCall{
-			Name: funcs.OperatorFuncName,
+			Name: funcs.ConcatFuncName,
 			Args: []interfaces.Expr{
-				&ast.ExprStr{
-					V: "+",
-				},
 				&ast.ExprStr{
 					V: "i am: ",
 				},
@@ -696,12 +675,9 @@ func TestInterpolateBasicExpr(t *testing.T) {
 	//		V: "i am: ${blah(21, -12.3)}",
 	//	}
 	//	exp := &ast.ExprCall{
-	//		Name: funcs.OperatorFuncName,
+	//		Name: funcs.ConcatFuncName,
 	//		Args: []interfaces.Expr{
 	//			&ast.ExprStr{
-	//				V: "+",
-	//			},
-	//			&ast.ExprStr{
 	//				V: "i am: ",
 	//			},
 	//			&ast.ExprCall{
@@ -730,12 +706,9 @@ func TestInterpolateBasicExpr(t *testing.T) {
 	//		V: "sweetie${-3.14159}", // FIXME: only the negative breaks this
 	//	}
 	//	exp := &ast.ExprCall{
-	//		Name: funcs.OperatorFuncName,
+	//		Name: funcs.ConcatFuncName,
 	//		Args: []interfaces.Expr{
 	//			&ast.ExprStr{
-	//				V: "+",
-	//			},
-	//			&ast.ExprStr{
 	//				V: "sweetie",
 	//			},
 	//			&ast.ExprFloat{
@@ -756,12 +729,9 @@ func TestInterpolateBasicExpr(t *testing.T) {
 	//		V: `i am: ${blah(42, "${foo}")}`,
 	//	}
 	//	exp := &ast.ExprCall{
-	//		Name: funcs.OperatorFuncName,
+	//		Name: funcs.ConcatFuncName,
 	//		Args: []interfaces.Expr{
 	//			&ast.ExprStr{
-	//				V: "+",
-	//			},
-	//			&ast.ExprStr{
 	//				V: "i am: ",
 	//			},
 	//			&ast.ExprCall{