fix: no stringify in hot loop

Makefile

@@ -13,6 +13,9 @@ thunk: it
 saccharine: it
 	@ ./lambda.exe - < ./samples/saccharine.txt
 
+church: it
+	@ ./lambda.exe - < ./samples/church.txt
+
 prof:
 	@ go tool pprof -top profile/cpu.prof
 

cmd/lambda/lambda.go

@@ -41,10 +41,14 @@ func main() {
 	// Turn tokens into syntax tree.
 	expression, err := saccharine.Parse(tokens)
 	cli.HandleError(err)
+	if options.Verbose {
 		logger.Info("parsed syntax tree", "tree", saccharine.Stringify(expression))
+	}
 
 	compiled := convert.SaccharineToLambda(expression)
+	if options.Verbose {
 		logger.Info("compiled lambda expression", "tree", lambda.Stringify(compiled))
+	}
 
 	// Reduce expression.
 	start := time.Now()
@@ -56,7 +60,9 @@ func main() {
 	steps := 0
 
 	for lambda.ReduceOnce(&compiled) {
+		if options.Verbose {
 			logger.Info("reduction", "tree", lambda.Stringify(compiled))
+		}
 		if options.Explanation {
 			fmt.Println("    =", lambda.Stringify(compiled))
 		}

pkg/lambda/substitute.go

@@ -11,16 +11,14 @@ func Substitute(e *Expression, target string, replacement Expression) {
 			return
 		}
 
-		if !IsFreeVariable(typed.Parameter, replacement) {
+		if IsFreeVariable(typed.Parameter, replacement) {
-			Substitute(&typed.Body, target, replacement)
-			return
-		}
-
 			replacementFreeVars := GetFreeVariables(replacement)
 			used := GetFreeVariables(typed.Body)
 			used.Merge(replacementFreeVars)
 			freshVar := GenerateFreshName(used)
 			Rename(typed, typed.Parameter, freshVar)
+		}
+
 		Substitute(&typed.Body, target, replacement)
 	case *Application:
 		Substitute(&typed.Abstraction, target, replacement)

samples/church.txt

@@ -0,0 +1,7 @@
+0 := \f.\x.x
+inc n := \f x.(f (n f x))
+exp n m := (m n)
+
+N := (inc (inc (inc (inc (inc 0)))))
+
+(exp N N)