Move value-only methods to `InstallableValue`

These methods would previously fail on the other `Installable`s, so
moving them to this class is more correct as to where they actually
work.

Additionally, a `InstallableValueCommand` is created to make it easier
(or rather no worse than before) to write commands that just work on
`InstallableValue`s.

Besides being a cleanup to avoid failing default methods, this gets us
closer to https://github.com/NixOS/rfcs/pull/134.

14 files changed, 127 insertions, 71 deletions

diff --git a/src/libcmd/command-installable-value.cc b/src/libcmd/command-installable-value.cc
new file mode 100644
index 000000000..d7581534b
--- /dev/null
+++ b/src/libcmd/command-installable-value.cc
@@ -0,0 +1,11 @@
+#include "command-installable-value.hh"
+
+namespace nix {
+
+void InstallableValueCommand::run(ref<Store> store, ref<Installable> installable)
+{
+	auto installableValue = InstallableValue::require(installable);
+	run(store, installableValue);
+}
+
+}
diff --git a/src/libcmd/command-installable-value.hh b/src/libcmd/command-installable-value.hh
new file mode 100644
index 000000000..8e31a0b92
--- /dev/null
+++ b/src/libcmd/command-installable-value.hh
@@ -0,0 +1,13 @@
+#include "installable-value.hh"
+#include "command.hh"
+
+namespace nix {
+
+struct InstallableValueCommand : InstallableCommand
+{
+    virtual void run(ref<Store> store, ref<InstallableValue> installable) = 0;
+
+    void run(ref<Store> store, ref<Installable> installable) override;
+};
+
+}
diff --git a/src/libcmd/installable-value.cc b/src/libcmd/installable-value.cc
new file mode 100644
index 000000000..30f80edb2
--- /dev/null
+++ b/src/libcmd/installable-value.cc
@@ -0,0 +1,44 @@
+#include "installable-value.hh"
+#include "eval-cache.hh"
+
+namespace nix {
+
+std::vector<ref<eval_cache::AttrCursor>>
+InstallableValue::getCursors(EvalState & state)
+{
+    auto evalCache =
+        std::make_shared<nix::eval_cache::EvalCache>(std::nullopt, state,
+            [&]() { return toValue(state).first; });
+    return {evalCache->getRoot()};
+}
+
+ref<eval_cache::AttrCursor>
+InstallableValue::getCursor(EvalState & state)
+{
+    /* Although getCursors should return at least one element, in case it doesn't,
+       bound check to avoid an undefined behavior for vector[0] */
+    return getCursors(state).at(0);
+}
+
+static UsageError nonValueInstallable(Installable & installable)
+{
+	return UsageError("installable '%s' does not correspond to a Nix language value", installable.what());
+}
+
+InstallableValue & InstallableValue::require(Installable & installable)
+{
+    auto * castedInstallable = dynamic_cast<InstallableValue *>(&installable);
+    if (!castedInstallable)
+        throw nonValueInstallable(installable);
+    return *castedInstallable;
+}
+
+ref<InstallableValue> InstallableValue::require(ref<Installable> installable)
+{
+    auto castedInstallable = installable.dynamic_pointer_cast<InstallableValue>();
+    if (!castedInstallable)
+        throw nonValueInstallable(*installable);
+    return ref { castedInstallable };
+}
+
+}
diff --git a/src/libcmd/installable-value.hh b/src/libcmd/installable-value.hh
index c6cdc4797..682c8d942 100644
--- a/src/libcmd/installable-value.hh
+++ b/src/libcmd/installable-value.hh
@@ -4,11 +4,41 @@
 
 namespace nix {
 
+struct App
+{
+    std::vector<DerivedPath> context;
+    Path program;
+    // FIXME: add args, sandbox settings, metadata, ...
+};
+
+struct UnresolvedApp
+{
+    App unresolved;
+    App resolve(ref<Store> evalStore, ref<Store> store);
+};
+
 struct InstallableValue : Installable
 {
     ref<EvalState> state;
 
     InstallableValue(ref<EvalState> state) : state(state) {}
+
+    virtual std::pair<Value *, PosIdx> toValue(EvalState & state) = 0;
+
+    /* Get a cursor to each value this Installable could refer to. However
+       if none exists, throw exception instead of returning empty vector. */
+    virtual std::vector<ref<eval_cache::AttrCursor>>
+    getCursors(EvalState & state);
+
+    /* Get the first and most preferred cursor this Installable could refer
+       to, or throw an exception if none exists. */
+    virtual ref<eval_cache::AttrCursor>
+    getCursor(EvalState & state);
+
+    UnresolvedApp toApp(EvalState & state);
+
+    static InstallableValue & require(Installable & installable);
+    static ref<InstallableValue> require(ref<Installable> installable);
 };
 
 }
diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 5cbf26b88..e2164ec72 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -364,23 +364,6 @@ DerivedPathWithInfo Installable::toDerivedPath()
     return std::move(buildables[0]);
 }
 
-std::vector<ref<eval_cache::AttrCursor>>
-Installable::getCursors(EvalState & state)
-{
-    auto evalCache =
-        std::make_shared<nix::eval_cache::EvalCache>(std::nullopt, state,
-            [&]() { return toValue(state).first; });
-    return {evalCache->getRoot()};
-}
-
-ref<eval_cache::AttrCursor>
-Installable::getCursor(EvalState & state)
-{
-    /* Although getCursors should return at least one element, in case it doesn't,
-       bound check to avoid an undefined behavior for vector[0] */
-    return getCursors(state).at(0);
-}
-
 static StorePath getDeriver(
     ref<Store> store,
     const Installable & i,
diff --git a/src/libcmd/installables.hh b/src/libcmd/installables.hh
index 6c2922d89..c5f3cd683 100644
--- a/src/libcmd/installables.hh
+++ b/src/libcmd/installables.hh
@@ -18,19 +18,6 @@ struct SourceExprCommand;
 
 namespace eval_cache { class EvalCache; class AttrCursor; }
 
-struct App
-{
-    std::vector<DerivedPath> context;
-    Path program;
-    // FIXME: add args, sandbox settings, metadata, ...
-};
-
-struct UnresolvedApp
-{
-    App unresolved;
-    App resolve(ref<Store> evalStore, ref<Store> store);
-};
-
 enum class Realise {
     /* Build the derivation. Postcondition: the
        derivation outputs exist. */
@@ -92,13 +79,6 @@ struct Installable
 
     DerivedPathWithInfo toDerivedPath();
 
-    UnresolvedApp toApp(EvalState & state);
-
-    virtual std::pair<Value *, PosIdx> toValue(EvalState & state)
-    {
-        throw Error("argument '%s' cannot be evaluated", what());
-    }
-
     /* Return a value only if this installable is a store path or a
        symlink to it. */
     virtual std::optional<StorePath> getStorePath()
@@ -106,16 +86,6 @@ struct Installable
         return {};
     }
 
-    /* Get a cursor to each value this Installable could refer to. However
-       if none exists, throw exception instead of returning empty vector. */
-    virtual std::vector<ref<eval_cache::AttrCursor>>
-    getCursors(EvalState & state);
-
-    /* Get the first and most preferred cursor this Installable could refer
-       to, or throw an exception if none exists. */
-    virtual ref<eval_cache::AttrCursor>
-    getCursor(EvalState & state);
-
     virtual FlakeRef nixpkgsFlakeRef() const
     {
         return FlakeRef::fromAttrs({{"type","indirect"}, {"id", "nixpkgs"}});
diff --git a/src/nix/app.cc b/src/nix/app.cc
index bfd75e278..fd4569bb4 100644
--- a/src/nix/app.cc
+++ b/src/nix/app.cc
@@ -1,5 +1,6 @@
 #include "installables.hh"
 #include "installable-derived-path.hh"
+#include "installable-value.hh"
 #include "store-api.hh"
 #include "eval-inline.hh"
 #include "eval-cache.hh"
@@ -40,7 +41,7 @@ std::string resolveString(
     return rewriteStrings(toResolve, rewrites);
 }
 
-UnresolvedApp Installable::toApp(EvalState & state)
+UnresolvedApp InstallableValue::toApp(EvalState & state)
 {
     auto cursor = getCursor(state);
     auto attrPath = cursor->getAttrPath();
diff --git a/src/nix/bundle.cc b/src/nix/bundle.cc
index 973bbd423..7c32a360e 100644
--- a/src/nix/bundle.cc
+++ b/src/nix/bundle.cc
@@ -1,5 +1,5 @@
-#include "command.hh"
 #include "installable-flake.hh"
+#include "command-installable-value.hh"
 #include "common-args.hh"
 #include "shared.hh"
 #include "store-api.hh"
@@ -8,7 +8,7 @@
 
 using namespace nix;
 
-struct CmdBundle : InstallableCommand
+struct CmdBundle : InstallableValueCommand
 {
     std::string bundler = "github:NixOS/bundlers";
     std::optional<Path> outLink;
@@ -70,7 +70,7 @@ struct CmdBundle : InstallableCommand
         return res;
     }
 
-    void run(ref<Store> store, ref<Installable> installable) override
+    void run(ref<Store> store, ref<InstallableValue> installable) override
     {
         auto evalState = getEvalState();
 
diff --git a/src/nix/edit.cc b/src/nix/edit.cc
index c46c1c23c..66629fab0 100644
--- a/src/nix/edit.cc
+++ b/src/nix/edit.cc
@@ -1,4 +1,4 @@
-#include "command.hh"
+#include "command-installable-value.hh"
 #include "shared.hh"
 #include "eval.hh"
 #include "attr-path.hh"
@@ -9,7 +9,7 @@
 
 using namespace nix;
 
-struct CmdEdit : InstallableCommand
+struct CmdEdit : InstallableValueCommand
 {
     std::string description() override
     {
@@ -25,7 +25,7 @@ struct CmdEdit : InstallableCommand
 
     Category category() override { return catSecondary; }
 
-    void run(ref<Store> store, ref<Installable> installable) override
+    void run(ref<Store> store, ref<InstallableValue> installable) override
     {
         auto state = getEvalState();
 
diff --git a/src/nix/eval.cc b/src/nix/eval.cc
index 6c2b60427..43db5150c 100644
--- a/src/nix/eval.cc
+++ b/src/nix/eval.cc
@@ -1,4 +1,4 @@
-#include "command.hh"
+#include "command-installable-value.hh"
 #include "common-args.hh"
 #include "shared.hh"
 #include "store-api.hh"
@@ -11,13 +11,13 @@
 
 using namespace nix;
 
-struct CmdEval : MixJSON, InstallableCommand, MixReadOnlyOption
+struct CmdEval : MixJSON, InstallableValueCommand, MixReadOnlyOption
 {
     bool raw = false;
     std::optional<std::string> apply;
     std::optional<Path> writeTo;
 
-    CmdEval() : InstallableCommand()
+    CmdEval() : InstallableValueCommand()
     {
         addFlag({
             .longName = "raw",
@@ -54,7 +54,7 @@ struct CmdEval : MixJSON, InstallableCommand, MixReadOnlyOption
 
     Category category() override { return catSecondary; }
 
-    void run(ref<Store> store, ref<Installable> installable) override
+    void run(ref<Store> store, ref<InstallableValue> installable) override
     {
         if (raw && json)
             throw UsageError("--raw and --json are mutually exclusive");
diff --git a/src/nix/fmt.cc b/src/nix/fmt.cc
index 6f6a4a632..c85eacded 100644
--- a/src/nix/fmt.cc
+++ b/src/nix/fmt.cc
@@ -1,4 +1,5 @@
 #include "command.hh"
+#include "installable-value.hh"
 #include "run.hh"
 
 using namespace nix;
@@ -31,8 +32,9 @@ struct CmdFmt : SourceExprCommand {
         auto evalState = getEvalState();
         auto evalStore = getEvalStore();
 
-        auto installable = parseInstallable(store, ".");
-        auto app = installable->toApp(*evalState).resolve(evalStore, store);
+        auto installable_ = parseInstallable(store, ".");
+        auto & installable = InstallableValue::require(*installable_);
+        auto app = installable.toApp(*evalState).resolve(evalStore, store);
 
         Strings programArgs{app.program};
 
diff --git a/src/nix/repl.cc b/src/nix/repl.cc
index 51d3074b4..c2d77ad5f 100644
--- a/src/nix/repl.cc
+++ b/src/nix/repl.cc
@@ -1,6 +1,7 @@
 #include "eval.hh"
 #include "globals.hh"
 #include "command.hh"
+#include "installable-value.hh"
 #include "repl.hh"
 
 namespace nix {
@@ -57,11 +58,12 @@ struct CmdRepl : RawInstallablesCommand
         auto getValues = [&]()->AbstractNixRepl::AnnotatedValues{
             auto installables = parseInstallables(store, rawInstallables);
             AbstractNixRepl::AnnotatedValues values;
-            for (auto & installable: installables){
-                auto what = installable->what();
+            for (auto & installable_: installables){
+                auto & installable = InstallableValue::require(*installable_);
+                auto what = installable.what();
                 if (file){
-                    auto [val, pos] = installable->toValue(*state);
-                    auto what = installable->what();
+                    auto [val, pos] = installable.toValue(*state);
+                    auto what = installable.what();
                     state->forceValue(*val, pos);
                     auto autoArgs = getAutoArgs(*state);
                     auto valPost = state->allocValue();
@@ -69,7 +71,7 @@ struct CmdRepl : RawInstallablesCommand
                     state->forceValue(*valPost, pos);
                     values.push_back( {valPost, what });
                 } else {
-                    auto [val, pos] = installable->toValue(*state);
+                    auto [val, pos] = installable.toValue(*state);
                     values.push_back( {val, what} );
                 }
             }
diff --git a/src/nix/run.cc b/src/nix/run.cc
index 56605d9d5..1baf299ab 100644
--- a/src/nix/run.cc
+++ b/src/nix/run.cc
@@ -1,5 +1,5 @@
 #include "run.hh"
-#include "command.hh"
+#include "command-installable-value.hh"
 #include "common-args.hh"
 #include "shared.hh"
 #include "store-api.hh"
@@ -137,7 +137,7 @@ struct CmdShell : InstallablesCommand, MixEnvironment
 
 static auto rCmdShell = registerCommand<CmdShell>("shell");
 
-struct CmdRun : InstallableCommand
+struct CmdRun : InstallableValueCommand
 {
     using InstallableCommand::run;
 
@@ -183,7 +183,7 @@ struct CmdRun : InstallableCommand
         return res;
     }
 
-    void run(ref<Store> store, ref<Installable> installable) override
+    void run(ref<Store> store, ref<InstallableValue> installable) override
     {
         auto state = getEvalState();
 
diff --git a/src/nix/search.cc b/src/nix/search.cc
index 994ec44c2..c92ed1663 100644
--- a/src/nix/search.cc
+++ b/src/nix/search.cc
@@ -1,4 +1,4 @@
-#include "command.hh"
+#include "command-installable-value.hh"
 #include "globals.hh"
 #include "eval.hh"
 #include "eval-inline.hh"
@@ -22,7 +22,7 @@ std::string wrap(std::string prefix, std::string s)
     return concatStrings(prefix, s, ANSI_NORMAL);
 }
 
-struct CmdSearch : InstallableCommand, MixJSON
+struct CmdSearch : InstallableValueCommand, MixJSON
 {
     std::vector<std::string> res;
     std::vector<std::string> excludeRes;
@@ -61,7 +61,7 @@ struct CmdSearch : InstallableCommand, MixJSON
         };
     }
 
-    void run(ref<Store> store, ref<Installable> installable) override
+    void run(ref<Store> store, ref<InstallableValue> installable) override
     {
         settings.readOnlyMode = true;
         evalSettings.enableImportFromDerivation.setDefault(false);