Split `OptionalPathSetting` from `PathSetting`

Rather than doing `allowEmpty` as boolean, have separate types and use
`std::optional`. This makes it harder to forget the possibility of an
empty path.

The `build-hook` setting was categorized as a `PathSetting`, but
actually it was split into arguments. No good! Now, it is
`Setting<Strings>` which actually reflects what it means and how it is
used.

Because of the subtyping, we now also have support for
`Setting<std::optional<String>>` in general. I imagine this can be used
to clean up many more settings also.

4 files changed, 111 insertions, 45 deletions

diff --git a/src/libutil/abstract-setting-to-json.hh b/src/libutil/abstract-setting-to-json.hh
index 7b6c3fcb5..d506dfb74 100644
--- a/src/libutil/abstract-setting-to-json.hh
+++ b/src/libutil/abstract-setting-to-json.hh
@@ -3,6 +3,7 @@
 
 #include <nlohmann/json.hpp>
 #include "config.hh"
+#include "json-utils.hh"
 
 namespace nix {
 template<typename T>
diff --git a/src/libutil/config-impl.hh b/src/libutil/config-impl.hh
index b6cae5ec3..14822050e 100644
--- a/src/libutil/config-impl.hh
+++ b/src/libutil/config-impl.hh
@@ -50,8 +50,11 @@ template<> void BaseSetting<std::set<ExperimentalFeature>>::appendOrSet(std::set
 template<typename T>
 void BaseSetting<T>::appendOrSet(T && newValue, bool append)
 {
-    static_assert(!trait::appendable, "using default `appendOrSet` implementation with an appendable type");
+    static_assert(
+        !trait::appendable,
+        "using default `appendOrSet` implementation with an appendable type");
     assert(!append);
+
     value = std::move(newValue);
 }
 
@@ -68,4 +71,60 @@ void BaseSetting<T>::set(const std::string & str, bool append)
     }
 }
 
+template<> void BaseSetting<bool>::convertToArg(Args & args, const std::string & category);
+
+template<typename T>
+void BaseSetting<T>::convertToArg(Args & args, const std::string & category)
+{
+    args.addFlag({
+        .longName = name,
+        .description = fmt("Set the `%s` setting.", name),
+        .category = category,
+        .labels = {"value"},
+        .handler = {[this](std::string s) { overridden = true; set(s); }},
+        .experimentalFeature = experimentalFeature,
+    });
+
+    if (isAppendable())
+        args.addFlag({
+            .longName = "extra-" + name,
+            .description = fmt("Append to the `%s` setting.", name),
+            .category = category,
+            .labels = {"value"},
+            .handler = {[this](std::string s) { overridden = true; set(s, true); }},
+            .experimentalFeature = experimentalFeature,
+        });
+}
+
+#define DECLARE_CONFIG_SERIALISER(TY) \
+    template<> TY BaseSetting< TY >::parse(const std::string & str) const; \
+    template<> std::string BaseSetting< TY >::to_string() const;
+
+DECLARE_CONFIG_SERIALISER(std::string)
+DECLARE_CONFIG_SERIALISER(std::optional<std::string>)
+DECLARE_CONFIG_SERIALISER(bool)
+DECLARE_CONFIG_SERIALISER(Strings)
+DECLARE_CONFIG_SERIALISER(StringSet)
+DECLARE_CONFIG_SERIALISER(StringMap)
+DECLARE_CONFIG_SERIALISER(std::set<ExperimentalFeature>)
+
+template<typename T>
+T BaseSetting<T>::parse(const std::string & str) const
+{
+    static_assert(std::is_integral<T>::value, "Integer required.");
+
+    if (auto n = string2Int<T>(str))
+        return *n;
+    else
+        throw UsageError("setting '%s' has invalid value '%s'", name, str);
+}
+
+template<typename T>
+std::string BaseSetting<T>::to_string() const
+{
+    static_assert(std::is_integral<T>::value, "Integer required.");
+
+    return std::to_string(value);
+}
+
 }
diff --git a/src/libutil/config.cc b/src/libutil/config.cc
index 085a884dc..38d406e8a 100644
--- a/src/libutil/config.cc
+++ b/src/libutil/config.cc
@@ -219,29 +219,6 @@ void AbstractSetting::convertToArg(Args & args, const std::string & category)
 {
 }
 
-template<typename T>
-void BaseSetting<T>::convertToArg(Args & args, const std::string & category)
-{
-    args.addFlag({
-        .longName = name,
-        .description = fmt("Set the `%s` setting.", name),
-        .category = category,
-        .labels = {"value"},
-        .handler = {[this](std::string s) { overridden = true; set(s); }},
-        .experimentalFeature = experimentalFeature,
-    });
-
-    if (isAppendable())
-        args.addFlag({
-            .longName = "extra-" + name,
-            .description = fmt("Append to the `%s` setting.", name),
-            .category = category,
-            .labels = {"value"},
-            .handler = {[this](std::string s) { overridden = true; set(s, true); }},
-            .experimentalFeature = experimentalFeature,
-        });
-}
-
 template<> std::string BaseSetting<std::string>::parse(const std::string & str) const
 {
     return str;
@@ -252,21 +229,17 @@ template<> std::string BaseSetting<std::string>::to_string() const
     return value;
 }
 
-template<typename T>
-T BaseSetting<T>::parse(const std::string & str) const
+template<> std::optional<std::string> BaseSetting<std::optional<std::string>>::parse(const std::string & str) const
 {
-    static_assert(std::is_integral<T>::value, "Integer required.");
-    if (auto n = string2Int<T>(str))
-        return *n;
+    if (str == "")
+        return std::nullopt;
     else
-        throw UsageError("setting '%s' has invalid value '%s'", name, str);
+        return { str };
 }
 
-template<typename T>
-std::string BaseSetting<T>::to_string() const
+template<> std::string BaseSetting<std::optional<std::string>>::to_string() const
 {
-    static_assert(std::is_integral<T>::value, "Integer required.");
-    return std::to_string(value);
+    return value ? *value : "";
 }
 
 template<> bool BaseSetting<bool>::parse(const std::string & str) const
@@ -403,17 +376,27 @@ template class BaseSetting<StringSet>;
 template class BaseSetting<StringMap>;
 template class BaseSetting<std::set<ExperimentalFeature>>;
 
-Path PathSetting::parse(const std::string & str) const
+static Path parsePath(const AbstractSetting & s, const std::string & str)
 {
-    if (str == "") {
-        if (allowEmpty)
-            return "";
-        else
-            throw UsageError("setting '%s' cannot be empty", name);
-    } else
+    if (str == "")
+        throw UsageError("setting '%s' is a path and paths cannot be empty", s.name);
+    else
         return canonPath(str);
 }
 
+Path PathSetting::parse(const std::string & str) const
+{
+    return parsePath(*this, str);
+}
+
+std::optional<Path> OptionalPathSetting::parse(const std::string & str) const
+{
+    if (str == "")
+        return std::nullopt;
+    else
+        return parsePath(*this, str);
+}
+
 bool GlobalConfig::set(const std::string & name, const std::string & value)
 {
     for (auto & config : *configRegistrations)
diff --git a/src/libutil/config.hh b/src/libutil/config.hh
index 2675baed7..cc8532587 100644
--- a/src/libutil/config.hh
+++ b/src/libutil/config.hh
@@ -353,21 +353,20 @@ public:
 /**
  * A special setting for Paths. These are automatically canonicalised
  * (e.g. "/foo//bar/" becomes "/foo/bar").
+ *
+ * It is mandatory to specify a path; i.e. the empty string is not
+ * permitted.
  */
 class PathSetting : public BaseSetting<Path>
 {
-    bool allowEmpty;
-
 public:
 
     PathSetting(Config * options,
-        bool allowEmpty,
         const Path & def,
         const std::string & name,
         const std::string & description,
         const std::set<std::string> & aliases = {})
         : BaseSetting<Path>(def, true, name, description, aliases)
-        , allowEmpty(allowEmpty)
     {
         options->addSetting(this);
     }
@@ -379,6 +378,30 @@ public:
     void operator =(const Path & v) { this->assign(v); }
 };
 
+/**
+ * Like `PathSetting`, but the absence of a path is also allowed.
+ *
+ * `std::optional` is used instead of the empty string for clarity.
+ */
+class OptionalPathSetting : public BaseSetting<std::optional<Path>>
+{
+public:
+
+    OptionalPathSetting(Config * options,
+        const std::optional<Path> & def,
+        const std::string & name,
+        const std::string & description,
+        const std::set<std::string> & aliases = {})
+        : BaseSetting<std::optional<Path>>(def, true, name, description, aliases)
+    {
+        options->addSetting(this);
+    }
+
+    std::optional<Path> parse(const std::string & str) const override;
+
+    void operator =(const std::optional<Path> & v) { this->assign(v); }
+};
+
 struct GlobalConfig : public AbstractConfig
 {
     typedef std::vector<Config*> ConfigRegistrations;