C API: Add nix_value_call_multi, NIX_VALUE_CALL

_multi can be implemented more efficiently.
NIX_VALUE_CALL is a convenient way to invoke it.

src/libexpr-c/nix_api_expr.cc

@@ -65,6 +65,17 @@ nix_err nix_value_call(nix_c_context * context, EvalState * state, Value * fn, V
     NIXC_CATCH_ERRS
 }
 
+nix_err nix_value_call_multi(nix_c_context * context, EvalState * state, Value * fn, size_t nargs, Value ** args, Value * value)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        state->state.callFunction(*(nix::Value *) fn, nargs, (nix::Value * *)args, *(nix::Value *) value, nix::noPos);
+        state->state.forceValue(*(nix::Value *) value, nix::noPos);
+    }
+    NIXC_CATCH_ERRS
+}
+
 nix_err nix_value_force(nix_c_context * context, EvalState * state, Value * value)
 {
     if (context)

src/libexpr-c/nix_api_expr.h

@@ -12,6 +12,7 @@
 
 #include "nix_api_store.h"
 #include "nix_api_util.h"
+#include <stddef.h>
 
 #ifdef __cplusplus
 extern "C" {
@@ -80,6 +81,46 @@ nix_err nix_expr_eval_from_string(
  */
 nix_err nix_value_call(nix_c_context * context, EvalState * state, Value * fn, Value * arg, Value * value);
 
+/**
+ * @brief Calls a Nix function with multiple arguments.
+ *
+ * Technically these are functions that return functions. It is common for Nix
+ * functions to be curried, so this function is useful for calling them.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] state The state of the evaluation.
+ * @param[in] fn The Nix function to call.
+ * @param[in] nargs The number of arguments.
+ * @param[in] args The arguments to pass to the function.
+ * @param[out] value The result of the function call.
+ *
+ * @see nix_value_call     For the single argument primitive.
+ * @see NIX_VALUE_CALL           For a macro that wraps this function for convenience.
+ */
+nix_err nix_value_call_multi(
+    nix_c_context * context, EvalState * state, Value * fn, size_t nargs, Value ** args, Value * value);
+
+/**
+ * @brief Calls a Nix function with multiple arguments.
+ *
+ * Technically these are functions that return functions. It is common for Nix
+ * functions to be curried, so this function is useful for calling them.
+ *
+ * @param[out] context Optional, stores error information
+ * @param[in] state The state of the evaluation.
+ * @param[out] value The result of the function call.
+ * @param[in] fn The Nix function to call.
+ * @param[in] args The arguments to pass to the function.
+ *
+ * @see nix_value_call_multi
+ */
+#define NIX_VALUE_CALL(context, state, value, fn, ...)                  \
+  do {                                                                  \
+    Value * args_array[] = {__VA_ARGS__};                               \
+    size_t nargs = sizeof(args_array) / sizeof(args_array[0]);          \
+    nix_value_call_multi(context, state, fn, nargs, args_array, value); \
+  } while (0)
+
 /**
  * @brief Forces the evaluation of a Nix value.
  *

tests/unit/libexpr/nix_api_expr.cc

@@ -250,7 +250,7 @@ static void primop_repeat(void * user_data, nix_c_context * context, EvalState *
     nix_init_string(context, ret, result.c_str());
 }
 
-TEST_F(nix_api_expr_test, nix_expr_primop_arity_2)
+TEST_F(nix_api_expr_test, nix_expr_primop_arity_2_multiple_calls)
 {
     PrimOp * primop =
         nix_alloc_primop(ctx, primop_repeat, 2, "repeat", nullptr, "repeat a string", (void *) SAMPLE_USER_DATA);
@@ -285,6 +285,38 @@ TEST_F(nix_api_expr_test, nix_expr_primop_arity_2)
     ASSERT_STREQ("hellohellohello", r.c_str());
 }
 
+TEST_F(nix_api_expr_test, nix_expr_primop_arity_2_single_call)
+{
+    PrimOp * primop =
+        nix_alloc_primop(ctx, primop_repeat, 2, "repeat", nullptr, "repeat a string", (void *) SAMPLE_USER_DATA);
+    assert_ctx_ok();
+    Value * primopValue = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_primop(ctx, primopValue, primop);
+    assert_ctx_ok();
+
+    Value * hello = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_string(ctx, hello, "hello");
+    assert_ctx_ok();
+
+    Value * three = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    nix_init_int(ctx, three, 3);
+    assert_ctx_ok();
+
+    Value * result = nix_alloc_value(ctx, state);
+    assert_ctx_ok();
+    NIX_VALUE_CALL(ctx, state, result, primopValue, hello, three);
+    assert_ctx_ok();
+
+    std::string r;
+    nix_get_string(ctx, result, OBSERVE_STRING(r));
+    assert_ctx_ok();
+
+    ASSERT_STREQ("hellohellohello", r.c_str());
+}
+
 static void
 primop_bad_no_return(void * user_data, nix_c_context * context, EvalState * state, Value ** args, Value * ret)
 {
@@ -357,4 +389,19 @@ TEST_F(nix_api_expr_test, nix_expr_primop_bad_return_thunk)
             testing::HasSubstr("Implementation error in custom function: return value must not be a thunk")));
     ASSERT_THAT(ctx->last_err, testing::Optional(testing::HasSubstr("badReturnThunk")));
 }
+
+TEST_F(nix_api_expr_test, nix_value_call_multi_no_args)
+{
+    Value * n = nix_alloc_value(ctx, state);
+    nix_init_int(ctx, n, 3);
+    assert_ctx_ok();
+
+    Value * r = nix_alloc_value(ctx, state);
+    nix_value_call_multi(ctx, state, n, 0, nullptr, r);
+    assert_ctx_ok();
+
+    auto rInt = nix_get_int(ctx, r);
+    assert_ctx_ok();
+    ASSERT_EQ(3, rInt);
+}
 } // namespace nixC