Add repl-overlays setting

doc/manual/rl-next/repl-overlays.md

@@ -0,0 +1,35 @@
+---
+synopsis: Add `repl-overlays` option
+prs: 10203
+---
+
+A `repl-overlays` option has been added, which specifies files that can overlay
+and modify the top-level bindings in `nix repl`. For example, with the
+following contents in `~/.config/nix/repl.nix`:
+
+```nix
+info: final: prev: let
+  optionalAttrs = predicate: attrs:
+    if predicate
+    then attrs
+    else {};
+in
+  optionalAttrs (prev ? legacyPackages && prev.legacyPackages ? ${info.currentSystem})
+  {
+    pkgs = prev.legacyPackages.${info.currentSystem};
+  }
+```
+
+We can run `nix repl` and use `pkgs` to refer to `legacyPackages.${currentSystem}`:
+
+```ShellSession
+$ nix repl --repl-overlays ~/.config/nix/repl.nix nixpkgs
+Nix 2.21.0pre20240309_4111bb6
+Type :? for help.
+Loading installable 'flake:nixpkgs#'...
+Added 5 variables.
+Loading 'repl-overlays'...
+Added 6 variables.
+nix-repl> pkgs.bash
+«derivation /nix/store/g08b5vkwwh0j8ic9rkmd8mpj878rk62z-bash-5.2p26.drv»
+```

src/libcmd/local.mk

@@ -13,3 +13,5 @@ libcmd_LDFLAGS = $(EDITLINE_LIBS) $(LOWDOWN_LIBS) $(THREAD_LDFLAGS)
 libcmd_LIBS = libstore libutil libexpr libmain libfetchers
 
 $(eval $(call install-file-in, $(buildprefix)$(d)/nix-cmd.pc, $(libdir)/pkgconfig, 0644))
+
+$(d)/repl.cc: $(d)/repl-overlays.nix.gen.hh

src/libcmd/repl-overlays.nix

@@ -0,0 +1,8 @@
+info:
+initial:
+functions:
+let final = builtins.foldl'
+              (prev: function: prev // (function info final prev))
+              initial
+              functions;
+in final

src/libcmd/repl.cc

@@ -43,7 +43,9 @@ extern "C" {
 #include "finally.hh"
 #include "markdown.hh"
 #include "local-fs-store.hh"
-#include "print.hh"
+#include "gc-small-vector.hh"
+#include "fs-input-accessor.hh"
+#include "memory-input-accessor.hh"
 
 #if HAVE_BOEHMGC
 #define GC_INCLUDE_NEW
@@ -114,6 +116,37 @@ struct NixRepl
     void evalString(std::string s, Value & v);
     void loadDebugTraceEnv(DebugTrace & dt);
 
+    /**
+     * Load the `repl-overlays` and add the resulting AttrSet to the top-level
+     * bindings.
+     */
+    void loadReplOverlays();
+
+    /**
+     * Get a list of each of the `repl-overlays` (parsed and evaluated).
+     */
+    Value * replOverlays();
+
+    /**
+     * Get the Nix function that composes the `repl-overlays` together.
+     */
+    Value * replOverlaysEvalFunction();
+
+    /**
+     * Get the `info` AttrSet that's passed as the first argument to each
+     * of the `repl-overlays`.
+     */
+    Value * replInitInfo();
+
+    /**
+     * Get the current top-level bindings as an AttrSet.
+     */
+    Value * bindingsToAttrs();
+    /**
+     * Parse a file, evaluate its result, and force the resulting value.
+     */
+    Value * evalFile(SourcePath & path);
+
     void printValue(std::ostream & str,
                               Value & v,
                               unsigned int maxDepth = std::numeric_limits<unsigned int>::max())
@@ -879,14 +912,83 @@ void NixRepl::loadFiles()
     loadedFiles.clear();
 
     for (auto & i : old) {
-        notice("Loading '%1%'...", i);
+        notice("Loading '%1%'...", Magenta(i));
         loadFile(i);
     }
 
     for (auto & [i, what] : getValues()) {
-        notice("Loading installable '%1%'...", what);
+        notice("Loading installable '%1%'...", Magenta(what));
         addAttrsToScope(*i);
     }
+
+    loadReplOverlays();
+}
+
+void NixRepl::loadReplOverlays()
+{
+    if (!evalSettings.replOverlays) {
+        return;
+    }
+
+    notice("Loading '%1%'...", Magenta("repl-overlays"));
+    auto replInitFilesFunction = replOverlaysEvalFunction();
+
+    Value &newAttrs(*state->allocValue());
+    SmallValueVector<3> args = {replInitInfo(), bindingsToAttrs(), replOverlays()};
+    state->callFunction(
+        *replInitFilesFunction,
+        args.size(),
+        args.data(),
+        newAttrs,
+        replInitFilesFunction->determinePos(noPos)
+    );
+
+    addAttrsToScope(newAttrs);
+}
+
+Value * NixRepl::replOverlaysEvalFunction()
+{
+    auto evalReplInitFilesPath = CanonPath("repl-overlays.nix");
+    if (!state->corepkgsFS->pathExists(evalReplInitFilesPath)) {
+        state->corepkgsFS->addFile(
+            evalReplInitFilesPath,
+            #include "repl-overlays.nix.gen.hh"
+        );
+    }
+
+    SourcePath evalReplInitFilesSourcePath(state->corepkgsFS, evalReplInitFilesPath);
+    return evalFile(evalReplInitFilesSourcePath);
+}
+
+Value * NixRepl::replOverlays()
+{
+    Value * replInits(state->allocValue());
+    state->mkList(*replInits, evalSettings.replOverlays.get().size());
+    Value ** replInitElems = replInits->listElems();
+
+    size_t i = 0;
+    for (auto path : evalSettings.replOverlays.get()) {
+        debug("Loading '%1%'...", path);
+        SourcePath sourcePath(makeFSInputAccessor(), CanonPath(path));
+        replInitElems[i] = evalFile(sourcePath);
+        i++;
+    }
+
+
+    return replInits;
+}
+
+Value * NixRepl::replInitInfo()
+{
+    auto builder = state->buildBindings(1);
+
+    Value * currentSystem(state->allocValue());
+    currentSystem->mkString(evalSettings.getCurrentSystem());
+    builder.insert(state->symbols.create("currentSystem"), currentSystem);
+
+    Value * info(state->allocValue());
+    info->mkAttrs(builder.finish());
+    return info;
 }
 
 
@@ -919,6 +1021,18 @@ void NixRepl::addVarToScope(const Symbol name, Value & v)
     varNames.emplace(state->symbols[name]);
 }
 
+Value * NixRepl::bindingsToAttrs()
+{
+    auto builder = state->buildBindings(staticEnv->vars.size());
+    for (auto & [symbol, displacement] : staticEnv->vars) {
+        builder.insert(symbol, env->values[displacement]);
+    }
+
+    Value * attrs(state->allocValue());
+    attrs->mkAttrs(builder.finish());
+    return attrs;
+}
+
 
 Expr * NixRepl::parseString(std::string s)
 {
@@ -933,6 +1047,15 @@ void NixRepl::evalString(std::string s, Value & v)
     state->forceValue(v, v.determinePos(noPos));
 }
 
+Value * NixRepl::evalFile(SourcePath & path)
+{
+    auto expr = state->parseExprFromFile(path, staticEnv);
+    Value * result(state->allocValue());
+    expr->eval(*state, *env, *result);
+    state->forceValue(*result, result->determinePos(noPos));
+    return result;
+}
+
 
 std::unique_ptr<AbstractNixRepl> AbstractNixRepl::create(
    const SearchPath & searchPath, nix::ref<Store> store, ref<EvalState> state,

src/libexpr/eval-settings.hh

@@ -137,6 +137,42 @@ struct EvalSettings : Config
 
           This is useful for debugging warnings in third-party Nix code.
         )"};
+
+    PathsSetting replOverlays{this, Paths(), "repl-overlays",
+        R"(
+          A list of files containing Nix expressions that can be used to add
+          default bindings to [`nix
+          repl`](@docroot@/command-ref/new-cli/nix3-repl.md) sessions.
+
+          Each file is called with three arguments:
+          1. An [attribute set](@docroot@/language/values.html#attribute-set)
+             containing a
+             [`currentSystem`](@docroot@/language/builtin-constants.md#builtins-currentSystem)
+             attribute (this is identical to
+             [`builtins.currentSystem`](@docroot@/language/builtin-constants.md#builtins-currentSystem),
+             except that it's available in
+             [`pure-eval`](@docroot@/command-ref/conf-file.html#conf-pure-eval)
+             mode).
+          2. The top-level bindings produced by the previous `repl-overlays`
+             value (or the default top-level bindings).
+          3. The final top-level bindings produced by calling all
+             `repl-overlays`.
+
+          For example, the following file would alias `pkgs` to
+          `legacyPackages.${info.currentSystem}` (if that attribute is defined):
+
+          ```nix
+          info: prev: final:
+          if prev ? legacyPackages
+             && prev.legacyPackages ? ${info.currentSystem}
+          then
+          {
+            pkgs = prev.legacyPackages.${info.currentSystem};
+          }
+          else
+          { }
+          ```
+        )"};
 };
 
 extern EvalSettings evalSettings;

src/libexpr/gc-small-vector.hh

@@ -4,6 +4,7 @@
 
 #if HAVE_BOEHMGC
 
+#define GC_INCLUDE_NEW
 #include <gc/gc.h>
 #include <gc/gc_cpp.h>
 #include <gc/gc_allocator.h>
@@ -39,4 +40,4 @@ using SmallValueVector = SmallVector<Value *, nItems>;
 template <size_t nItems>
 using SmallTemporaryValueVector = SmallVector<Value, nItems>;
 
-}
+}

src/libutil/config.cc

@@ -443,6 +443,39 @@ void OptionalPathSetting::operator =(const std::optional<Path> & v)
     this->assign(v);
 }
 
+PathsSetting::PathsSetting(Config * options,
+    const Paths & def,
+    const std::string & name,
+    const std::string & description,
+    const std::set<std::string> & aliases)
+    : BaseSetting<Paths>(def, true, name, description, aliases)
+{
+    options->addSetting(this);
+}
+
+
+Paths PathsSetting::parse(const std::string & str) const
+{
+    auto strings = tokenizeString<Strings>(str);
+    Paths parsed;
+
+    for (auto str : strings) {
+        parsed.push_back(canonPath(str));
+    }
+
+    return parsed;
+}
+
+void PathsSetting::operator =(const Paths & v)
+{
+    this->assign(v);
+}
+
+PathsSetting::operator bool() const noexcept
+{
+    return !get().empty();
+}
+
 bool GlobalConfig::set(const std::string & name, const std::string & value)
 {
     for (auto & config : *configRegistrations)

src/libutil/config.hh

@@ -375,6 +375,26 @@ public:
     void operator =(const std::optional<Path> & v);
 };
 
+/**
+ * Like `OptionalPathSetting`, but allows multiple paths.
+ */
+class PathsSetting : public BaseSetting<Paths>
+{
+public:
+
+    PathsSetting(Config * options,
+        const Paths & def,
+        const std::string & name,
+        const std::string & description,
+        const std::set<std::string> & aliases = {});
+
+    Paths parse(const std::string & str) const override;
+
+    void operator =(const Paths & v);
+
+    operator bool() const noexcept;
+};
+
 struct GlobalConfig : public AbstractConfig
 {
     typedef std::vector<Config*> ConfigRegistrations;