src/unstable-overlays/nix/5567-make-installables-expr-context.patch

From 519d541fdc71508793ece6e087d2c1a231378558 Mon Sep 17 00:00:00 2001
From: Skyler Grey <skyler3665@gmail.com>
Date: Wed, 20 Jul 2022 14:45:45 +0100
Subject: [PATCH 1/4] Make --expr always include nixpkgs

---
 src/libcmd/installables.cc | 7 +++++--
 1 file changed, 5 insertions(+), 2 deletions(-)

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 59162c4df03..7dc3f7435ee 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -778,8 +778,11 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
         } else if (file)
             state->evalFile(lookupFileArg(*state, *file), *vFile);
         else {
-            auto e = state->parseExprFromString(*expr, absPath("."));
-            state->eval(e, *vFile);
+            Strings e = {};
+            e.push_back("with (builtins.getFlake \"nixpkgs\")");
+            e.push_back(*expr);
+            auto parsed = state->parseExprFromString(concatStringsSep(";", e), absPath("."));
+            state->eval(parsed, *vFile);
         }
 
         for (auto & s : ss) {

From 29b83de5671901e5e8755c17b840139a6f8650db Mon Sep 17 00:00:00 2001
From: Skyler Grey <skyler3665@gmail.com>
Date: Wed, 20 Jul 2022 14:46:17 +0100
Subject: [PATCH 2/4] Stop --expr requiring --impure flag

---
 src/libcmd/installables.cc | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 7dc3f7435ee..43e759b3e93 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -767,7 +767,7 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
             throw UsageError("'--file' and '--expr' are exclusive");
 
         // FIXME: backward compatibility hack
-        if (file) evalSettings.pureEval = false;
+        evalSettings.pureEval = false;
 
         auto state = getEvalState();
         auto vFile = state->allocValue();

From 9e9d28496a7216a0e2893ac0eb05a8c45d898c95 Mon Sep 17 00:00:00 2001
From: Skyler Grey <skyler3665@gmail.com>
Date: Wed, 20 Jul 2022 23:51:16 +0100
Subject: [PATCH 3/4] Make nix shell take installables as context

---
 src/libcmd/installables.cc | 46 +++++++++++++++++++++++++++++++++++++-
 1 file changed, 45 insertions(+), 1 deletion(-)

diff --git a/src/libcmd/installables.cc b/src/libcmd/installables.cc
index 43e759b3e93..1abad72e333 100644
--- a/src/libcmd/installables.cc
+++ b/src/libcmd/installables.cc
@@ -779,10 +779,54 @@ std::vector<std::shared_ptr<Installable>> SourceExprCommand::parseInstallables(
             state->evalFile(lookupFileArg(*state, *file), *vFile);
         else {
             Strings e = {};
-            e.push_back("with (builtins.getFlake \"nixpkgs\")");
+
+            for (auto & s : ss) {
+                std::exception_ptr ex;
+
+                if (s.find('/') != std::string::npos) {
+                    try {
+                        result.push_back(std::make_shared<InstallableStorePath>(store, store->followLinksToStorePath(s)));
+                        continue;
+                    } catch (BadStorePath &) {
+                    } catch (...) {
+                        if (!ex)
+                            ex = std::current_exception();
+                    }
+                }
+
+                try {
+                    auto [flakeRef, fragment, outputsSpec] = parseFlakeRefWithFragmentAndOutputsSpec(s, absPath("."));
+/*                    result.push_back(std::make_shared<InstallableFlake>(
+                            this,
+                            getEvalState(),
+                            std::move(flakeRef),
+                            fragment,
+                            outputsSpec,
+                            getDefaultFlakeAttrPaths(),
+                            getDefaultFlakeAttrPathPrefixes(),
+                            lockFlags));*/
+                    e.push_back("with (builtins.getFlake \"" + flakeRef.to_string() + "\")" + (fragment != "" ? "." + fragment : ""));
+                    e.push_back("with (builtins.getFlake \"" + flakeRef.to_string() + "\").packages.\"${builtins.currentSystem}\"" + (fragment != "" ? "." + fragment : ""));
+                    e.push_back("with (builtins.getFlake \"" + flakeRef.to_string() + "\").legacyPackages.\"${builtins.currentSystem}\"" + (fragment != "" ? "." + fragment : ""));
+                    continue;
+                } catch (...) {
+                    ex = std::current_exception();
+                }
+
+                std::rethrow_exception(ex);
+            }
+
             e.push_back(*expr);
             auto parsed = state->parseExprFromString(concatStringsSep(";", e), absPath("."));
             state->eval(parsed, *vFile);
+
+            auto [prefix, outputsSpec] = parseOutputsSpec(".");
+            result.push_back(
+                std::make_shared<InstallableAttrPath>(
+                    state, *this, vFile,
+                    prefix == "." ? "" : prefix,
+                    outputsSpec));
+            return result;
         }
 
         for (auto & s : ss) {

From e3045be11cbac91a5e65ef42e42e320e6689e713 Mon Sep 17 00:00:00 2001
From: Skyler Grey <skyler3665@gmail.com>
Date: Wed, 20 Jul 2022 23:56:09 +0100
Subject: [PATCH 4/4] Update documentation to match nix shell change

---
 src/nix/nix.md | 9 +++++++--
 1 file changed, 7 insertions(+), 2 deletions(-)

diff --git a/src/nix/nix.md b/src/nix/nix.md
index d48682a9432..b5bee708451 100644
--- a/src/nix/nix.md
+++ b/src/nix/nix.md
@@ -139,8 +139,13 @@ the Nix store. Here are the recognised types of installables:
 * **Nix expressions**: `--expr '(import <nixpkgs> {}).hello.overrideDerivation (prev: { name = "my-hello"; })'`.
 
   When the `--expr` option is given, all installables are interpreted
-  as Nix expressions. You may need to specify `--impure` if the
-  expression references impure inputs (such as `<nixpkgs>`).
+  as context for the expression. For example, the following is valid:
+
+  ```console
+  # nix shell nixpkgs#python3 --expr 'withPackages(pyPkgs: with pyPkgs; [ numpy ])'
+  ```
+
+  Using `--expr` implies `--impure`.
 
 For most commands, if no installable is specified, the default is `.`,
 i.e. Nix will operate on the default flake output attribute of the