fable: Simplify several if-else statements

This was also a clang-tidy finding.

Author: cassava

fable/include/fable/environment.hpp

@@ -134,9 +134,8 @@ class Environment {
 inline std::string interpolate_vars(const std::string& s, const Environment* env = nullptr) {
   if (env != nullptr) {
     return env->interpolate(s);
-  } else {
-    return Environment().interpolate(s);
   }
+  return Environment().interpolate(s);
 }
 
 }  // namespace fable

fable/include/fable/schema/array.hpp

@@ -179,12 +179,14 @@ class Array : public Base<Array<T, N, P>> {
       return true;
     }
 
-    if (c->type() == JsonType::array) {
-      return this->validate_array(c, err);
-    } else if (c->type() == JsonType::object) {
-      return this->validate_object(c, err);
-    } else {
-      return this->set_wrong_type(err, c);
+    // If not require-all, then we can also support object notation.
+    switch (c->type()) {
+      case JsonType::array:
+        return this->validate_array(c, err);
+      case JsonType::object:
+        return this->validate_object(c, err);
+      default:
+        return this->set_wrong_type(err, c);
     }
   }
 

fable/include/fable/schema/factory.hpp

@@ -314,7 +314,7 @@ class FactoryBase : public Base<CRTP> {
 
     Conf args;
     if (args_subset_) {
-      if (args_key_ != "") {
+      if (!args_key_.empty()) {
         if (c.has(args_key_)) {
           args = c.at(args_key_);
         }
@@ -364,7 +364,7 @@ class FactoryBase : public Base<CRTP> {
       Struct base{
           {factory_key_, make_const_schema(kv.first, "name of factory").require()},
       };
-      if (args_key_ == "") {
+      if (args_key_.empty()) {
         base.set_properties_from(kv.second.schema);
       } else {
         base.set_property(args_key_, kv.second.schema.clone());

fable/include/fable/schema/interface.hpp

@@ -422,9 +422,8 @@ class Base : public Interface {
     const auto* required = required_ ? "!" : "";
     if (desc_.empty()) {
       return type_string() + required;
-    } else {
-      return fmt::format("{}{} :: {}", type_string(), required, desc_);
     }
+    return fmt::format("{}{} :: {}", type_string(), required, desc_);
   }
 
   [[nodiscard]] bool is_required() const override { return required_; }

fable/include/fable/schema/optional.hpp

@@ -119,11 +119,10 @@ class Optional : public Base<Optional<T, P>> {
   }
 
   [[nodiscard]] Json serialize(const Type& x) const {
-    if (x) {
-      return prototype_.serialize(x.value());
-    } else {
+    if (!x) {
       return nullptr;
     }
+    return prototype_.serialize(x.value());
   }
 
   [[nodiscard]] Type deserialize(const Conf& c) const {

fable/src/fable/conf.cpp

@@ -172,9 +172,8 @@ std::string Conf::resolve_file(const std::string& filepath) const {
 [[noreturn]] void Conf::throw_unexpected(const std::string& key, const std::string& msg) const {
   if (msg.empty()) {
     throw error::UnexpectedProperty(*this, key);
-  } else {
-    throw ConfError{*this, msg};
   }
+  throw ConfError{*this, msg};
 }
 
 [[noreturn]] void Conf::throw_missing(const std::string& key) const {
@@ -184,9 +183,8 @@ std::string Conf::resolve_file(const std::string& filepath) const {
 [[noreturn]] void Conf::throw_wrong_type(const std::string& key) const {
   if (key.empty()) {
     throw error::WrongType(*this);
-  } else {
-    throw error::WrongType(*this, key);
   }
+  throw error::WrongType(*this, key);
 }
 
 [[noreturn]] void Conf::throw_wrong_type(const std::string& key, JsonType type) const {