Merge "Add `repl-overlays`" into main

5 files changed, 214 insertions, 2 deletions

diff --git a/src/libcmd/local.mk b/src/libcmd/local.mk
index dcd33e84c..5d0e3ed7e 100644
--- a/src/libcmd/local.mk
+++ b/src/libcmd/local.mk
@@ -13,3 +13,5 @@ libcmd_LDFLAGS = $(EDITLINE_LIBS) $(LOWDOWN_LIBS) $(NIXDOC_LIBS) -pthread
 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
diff --git a/src/libcmd/meson.build b/src/libcmd/meson.build
index 867508926..b057daa37 100644
--- a/src/libcmd/meson.build
+++ b/src/libcmd/meson.build
@@ -32,8 +32,13 @@ libcmd_headers = files(
   'repl.hh',
 )
 
+libcmd_generated_headers = [
+  gen_header.process('repl-overlays.nix', preserve_path_from: meson.current_source_dir()),
+]
+
 libcmd = library(
   'nixcmd',
+  libcmd_generated_headers,
   libcmd_sources,
   dependencies : [
     liblixutil,
diff --git a/src/libcmd/repl-overlays.nix b/src/libcmd/repl-overlays.nix
new file mode 100644
index 000000000..33ce49482
--- /dev/null
+++ b/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
diff --git a/src/libcmd/repl.cc b/src/libcmd/repl.cc
index ee3a6e268..4aa46dbc0 100644
--- a/src/libcmd/repl.cc
+++ b/src/libcmd/repl.cc
@@ -30,6 +30,7 @@
 #include "signals.hh"
 #include "print.hh"
 #include "progress-bar.hh"
+#include "gc-small-vector.hh"
 
 #if HAVE_BOEHMGC
 #define GC_INCLUDE_NEW
@@ -118,6 +119,45 @@ 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 * getReplOverlaysEvalFunction();
+
+    /**
+     * Cached return value of `getReplOverlaysEvalFunction`.
+     *
+     * Note: This is `shared_ptr` to avoid garbage collection.
+     */
+    std::shared_ptr<Value *> replOverlaysEvalFunction =
+        std::allocate_shared<Value *>(traceable_allocator<Value *>(), nullptr);
+
+    /**
+     * 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())
@@ -770,14 +810,114 @@ 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 = getReplOverlaysEvalFunction();
+
+    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::getReplOverlaysEvalFunction()
+{
+    if (replOverlaysEvalFunction && *replOverlaysEvalFunction) {
+        return *replOverlaysEvalFunction;
+    }
+
+    auto evalReplInitFilesPath = CanonPath::root + "repl-overlays.nix";
+    *replOverlaysEvalFunction = state->allocValue();
+    auto code =
+        #include "repl-overlays.nix.gen.hh"
+        ;
+    auto expr = state->parseExprFromString(
+        code,
+        SourcePath(evalReplInitFilesPath),
+        state->staticBaseEnv
+    );
+
+    state->eval(expr, **replOverlaysEvalFunction);
+
+    return *replOverlaysEvalFunction;
+}
+
+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 '%2%'...", "repl-overlays", path);
+        SourcePath sourcePath((CanonPath(path)));
+        auto replInit = evalFile(sourcePath);
+
+        if (!replInit->isLambda()) {
+            state->error<TypeError>(
+                "Expected `repl-overlays` to be a lambda but found %1%: %2%",
+                showType(*replInit),
+                ValuePrinter(*state, *replInit, errorPrintOptions)
+            )
+            .atPos(replInit->determinePos(noPos))
+            .debugThrow();
+        }
+
+        if (replInit->lambda.fun->hasFormals()
+            && !replInit->lambda.fun->formals->ellipsis) {
+            state->error<TypeError>(
+                "Expected first argument of %1% to have %2% to allow future versions of Lix to add additional attributes to the argument",
+                "repl-overlays",
+                "..."
+            )
+                .atPos(replInit->determinePos(noPos))
+                .debugThrow();
+        }
+
+        replInitElems[i] = replInit;
+        i++;
+    }
+
+
+    return replInits;
+}
+
+Value * NixRepl::replInitInfo()
+{
+    auto builder = state->buildBindings(2);
+
+    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;
 }
 
 
@@ -810,6 +950,18 @@ 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)
 {
@@ -824,6 +976,15 @@ 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,
diff --git a/src/libexpr/eval-settings.hh b/src/libexpr/eval-settings.hh
index c264b521f..98fe6881e 100644
--- a/src/libexpr/eval-settings.hh
+++ b/src/libexpr/eval-settings.hh
@@ -125,6 +125,42 @@ 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 at least 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: final: prev:
+          if prev ? legacyPackages
+             && prev.legacyPackages ? ${info.currentSystem}
+          then
+          {
+            pkgs = prev.legacyPackages.${info.currentSystem};
+          }
+          else
+          { }
+          ```
+        )"};
 };
 
 extern EvalSettings evalSettings;