C API: Add nix_init_apply

Thunks are relevant when initializing attrsets and lists, passing
arguments. This is an important way to produce them.

src/libexpr-c/nix_api_expr.h

@@ -93,6 +93,8 @@ nix_err nix_expr_eval_from_string(
  * @param[in] arg The argument to pass to the function.
  * @param[out] value The result of the function call.
  * @return NIX_OK if the function call was successful, an error code otherwise.
+ * @see nix_init_apply() for a similar function that does not performs the call immediately, but stores it as a thunk.
+ *      Note the different argument order.
  */
 nix_err nix_value_call(nix_c_context * context, EvalState * state, Value * fn, Value * arg, Value * value);
 

src/libexpr-c/nix_api_value.cc

@@ -404,6 +404,19 @@ nix_err nix_init_null(nix_c_context * context, Value * value)
     NIXC_CATCH_ERRS
 }
 
+nix_err nix_init_apply(nix_c_context * context, Value * value, Value * fn, Value * arg)
+{
+    if (context)
+        context->last_err_code = NIX_OK;
+    try {
+        auto & v = check_value_not_null(value);
+        auto & f = check_value_not_null(fn);
+        auto & a = check_value_not_null(arg);
+        v.mkApp(&f, &a);
+    }
+    NIXC_CATCH_ERRS
+}
+
 nix_err nix_init_external(nix_c_context * context, Value * value, ExternalValue * val)
 {
     if (context)

src/libexpr-c/nix_api_value.h

@@ -342,8 +342,24 @@ nix_err nix_init_int(nix_c_context * context, Value * value, int64_t i);
  * @param[out] value Nix value to modify
  * @return error code, NIX_OK on success.
  */
-
 nix_err nix_init_null(nix_c_context * context, Value * value);
+
+/** @brief Set the value to a thunk that will perform a function application when needed.
+ *
+ * Thunks may be put into attribute sets and lists to perform some computation lazily; on demand.
+ * However, note that in some places, a thunk must not be returned, such as in the return value of a PrimOp.
+ * In such cases, you may use nix_value_call() instead (but note the different argument order).
+ *
+ * @param[out] context Optional, stores error information
+ * @param[out] value Nix value to modify
+ * @param[in] fn function to call
+ * @param[in] arg argument to pass
+ * @return error code, NIX_OK on successful initialization.
+ * @see nix_value_call() for a similar function that performs the call immediately and only stores the return value.
+ *      Note the different argument order.
+ */
+nix_err nix_init_apply(nix_c_context * context, Value * value, Value * fn, Value * arg);
+
 /** @brief Set an external value
  * @param[out] context Optional, stores error information
  * @param[out] value Nix value to modify
@@ -421,7 +437,7 @@ BindingsBuilder * nix_make_bindings_builder(nix_c_context * context, EvalState *
 /** @brief Insert bindings into a builder
  * @param[out] context Optional, stores error information
  * @param[in] builder BindingsBuilder to insert into
- * @param[in] name attribute name, copied into the symbol store
+ * @param[in] name attribute name, only used for the duration of the call.
  * @param[in] value value to give the binding
  * @return error code, NIX_OK on success.
  */

tests/unit/libexpr/nix_api_value.cc

@@ -8,6 +8,7 @@
 #include "tests/nix_api_expr.hh"
 #include "tests/string_callback.hh"
 
+#include "gmock/gmock.h"
 #include <cstdlib>
 #include <gtest/gtest.h>
 
@@ -187,4 +188,127 @@ TEST_F(nix_api_expr_test, nix_build_and_init_attr)
     free(out_name);
 }
 
+TEST_F(nix_api_expr_test, nix_value_init)
+{
+    // Setup
+
+    // two = 2;
+    // f = a: a * a;
+
+    Value * two = nix_alloc_value(ctx, state);
+    nix_init_int(ctx, two, 2);
+
+    Value * f = nix_alloc_value(ctx, state);
+    nix_expr_eval_from_string(
+        ctx, state, R"(
+        a: a * a
+    )",
+        "<test>", f);
+
+    // Test
+
+    // r = f two;
+
+    Value * r = nix_alloc_value(ctx, state);
+    nix_init_apply(ctx, r, f, two);
+    assert_ctx_ok();
+
+    ValueType t = nix_get_type(ctx, r);
+    assert_ctx_ok();
+
+    ASSERT_EQ(t, NIX_TYPE_THUNK);
+
+    nix_value_force(ctx, state, r);
+
+    t = nix_get_type(ctx, r);
+    assert_ctx_ok();
+
+    ASSERT_EQ(t, NIX_TYPE_INT);
+
+    int n = nix_get_int(ctx, r);
+    assert_ctx_ok();
+
+    ASSERT_EQ(n, 4);
+
+    // Clean up
+    nix_gc_decref(ctx, two);
+    nix_gc_decref(ctx, f);
+    nix_gc_decref(ctx, r);
+}
+
+TEST_F(nix_api_expr_test, nix_value_init_apply_error)
+{
+    Value * some_string = nix_alloc_value(ctx, state);
+    nix_init_string(ctx, some_string, "some string");
+    assert_ctx_ok();
+
+    Value * v = nix_alloc_value(ctx, state);
+    nix_init_apply(ctx, v, some_string, some_string);
+    assert_ctx_ok();
+
+    // All ok. Call has not been evaluated yet.
+
+    // Evaluate it
+    nix_value_force(ctx, state, v);
+    ASSERT_EQ(ctx->last_err_code, NIX_ERR_NIX_ERROR);
+    ASSERT_THAT(ctx->last_err.value(), testing::HasSubstr("attempt to call something which is not a function but"));
+
+    // Clean up
+    nix_gc_decref(ctx, some_string);
+    nix_gc_decref(ctx, v);
+}
+
+TEST_F(nix_api_expr_test, nix_value_init_apply_lazy_arg)
+{
+    // f is a lazy function: it does not evaluate its argument before returning its return value
+    // g is a helper to produce e
+    // e is a thunk that throws an exception
+    //
+    // r = f e
+    // r should not throw an exception, because e is not evaluated
+
+    Value * f = nix_alloc_value(ctx, state);
+    nix_expr_eval_from_string(
+        ctx, state, R"(
+        a: { foo = a; }
+    )",
+        "<test>", f);
+    assert_ctx_ok();
+
+    Value * e = nix_alloc_value(ctx, state);
+    {
+        Value * g = nix_alloc_value(ctx, state);
+        nix_expr_eval_from_string(
+            ctx, state, R"(
+            _ignore: throw "error message for test case nix_value_init_apply_lazy_arg"
+        )",
+            "<test>", g);
+        assert_ctx_ok();
+
+        nix_init_apply(ctx, e, g, g);
+        assert_ctx_ok();
+        nix_gc_decref(ctx, g);
+    }
+
+    Value * r = nix_alloc_value(ctx, state);
+    nix_init_apply(ctx, r, f, e);
+    assert_ctx_ok();
+
+    nix_value_force(ctx, state, r);
+    assert_ctx_ok();
+
+    auto n = nix_get_attrs_size(ctx, r);
+    assert_ctx_ok();
+    ASSERT_EQ(1, n);
+
+    // nix_get_attr_byname isn't lazy (it could have been) so it will throw the exception
+    Value * foo = nix_get_attr_byname(ctx, r, state, "foo");
+    ASSERT_EQ(nullptr, foo);
+    ASSERT_THAT(ctx->last_err.value(), testing::HasSubstr("error message for test case nix_value_init_apply_lazy_arg"));
+
+    // Clean up
+    nix_gc_decref(ctx, f);
+    nix_gc_decref(ctx, e);
+}
+
 }