type checking and error handling for builtins improved.

builtins/read.go

@@ -1,27 +1,25 @@
 package builtins
 
-import "monkey/object"
-
 import (
-	"os"
+	"io/ioutil"
+	"monkey/object"
+	"monkey/typing"
 )
 
 // Read ...
 func Read(args ...object.Object) object.Object {
-	if len(args) != 1 {
-		return newError("wrong number of arguments. got=%d, want=1",
-			len(args))
+	if err := typing.Check(
+		"read", args,
+		typing.ExactArgs(1),
+		typing.WithTypes(object.STRING_OBJ),
+	); err != nil {
+		return newError(err.Error())
 	}
 
-	arg, ok := args[0].(*object.String)
-	if !ok {
-		return newError("argument to `read` expected to be `str` got=%T", args[0].Type())
-	}
-
-	filename := arg.Value
-	data, err := os.ReadFile(filename)
+	filename := args[0].(*object.String).Value
+	data, err := ioutil.ReadFile(filename)
 	if err != nil {
-		return newError("error reading file: %s", err)
+		return newError("IOError: error reading from file %s: %s", filename, err)
 	}
 
 	return &object.String{Value: string(data)}