feature: Add experimental support for constructing Kernel exprs

.cargo/config.toml

@@ -1,2 +1,8 @@
 [alias]
 xtask = "run --package xtask --"
+
+# Avoid undefined symbol errors produced by the linker when calling functions
+# declared in wolfram-library-link/src/kernel/sys.rs that aren't resolved until
+# the built LibraryLink library is loaded into a running Kernel.
+[build]
+rustflags = ["-C", "link-args=-undefined dynamic_lookup"]

wolfram-library-link/Cargo.toml

@@ -44,6 +44,8 @@ nightly = []
 panic-failure-backtraces = ["backtrace"]
 automate-function-loading-boilerplate = ["inventory", "process_path", "wolfram-library-link-macros/automate-function-loading-boilerplate"]
 
+experimental-kernel-expr = []
+
 
 #=======================================
 # Examples

wolfram-library-link/RustLink/Tests/KernelExpr.wlt

@@ -0,0 +1,56 @@
+Needs["MUnit`"]
+
+Test[
+	(
+		LibraryFunctionLoad[
+			"liblibrary_tests", "test_kernel_expr_create_string", {}, "Void"
+		][];
+		Global`$ReturnValue
+	),
+	{1, "two", 3.5}
+]
+
+Test[
+	(
+		LibraryFunctionLoad[
+			"liblibrary_tests", "test_kernel_expr_create_symbols", {}, "Void"
+		][];
+		Global`$ReturnValue
+	),
+	{Global`Example1, Global`Example2, Example3`Example4}
+]
+
+Test[
+	(
+		LibraryFunctionLoad[
+			"liblibrary_tests", "test_kernel_expr_create_heterogenous", {}, "Void"
+		][];
+		Global`$ReturnValue
+	),
+	{1, 2.01, "three", Four, {"a", "b", "c"}}
+]
+
+Test[
+	(
+		LibraryFunctionLoad[
+			"liblibrary_tests", "test_kernel_expr_evaluate", {}, "Void"
+		][];
+		Global`$ReturnValue
+	),
+	4
+]
+
+(*====================================*)
+(* Custom DownCode                    *)
+(*====================================*)
+
+Test[
+	(
+		LibraryFunctionLoad[
+			"liblibrary_tests", "test_kernel_expr_custom_downcode", {}, "Void"
+		][];
+
+		Global`CustomDownCode[]
+	),
+	"CUSTOM DOWNCODE"
+]

wolfram-library-link/examples/tests/main.rs

@@ -6,3 +6,5 @@ mod test_data_store;
 mod test_images;
 mod test_numeric_array_conversions;
 mod test_wstp;
+
+mod test_kernel_expr;

wolfram-library-link/examples/tests/test_kernel_expr.rs

@@ -0,0 +1,77 @@
+use wolfram_library_link::{
+    export,
+    kernel::{self, Expr, MIntExpr, NormalExpr, SymbolExpr, UncountedExpr},
+};
+
+#[export]
+fn test_kernel_expr_create_string() {
+    let list = NormalExpr::list_from_array([
+        Expr::mint(1),
+        Expr::string("two"),
+        Expr::mreal(3.5),
+    ]);
+
+    // $ReturnValue = list
+    SymbolExpr::lookup("Global`$ReturnValue").set_to(&list.as_expr());
+}
+
+#[export]
+fn test_kernel_expr_create_symbols() {
+    let list = NormalExpr::list_from_array([
+        SymbolExpr::lookup("Example1").into(),
+        SymbolExpr::lookup("`Example2").into(),
+        SymbolExpr::lookup("Example3`Example4").into(),
+    ]);
+
+    // $ReturnValue = list
+    SymbolExpr::lookup("Global`$ReturnValue").set_to(&list.as_expr());
+}
+
+#[export]
+fn test_kernel_expr_create_heterogenous() {
+    let result = NormalExpr::list_from_array([
+        Expr::mint(1),
+        Expr::mreal(2.01),
+        Expr::string("three"),
+        Expr::symbol("Global`Four"),
+        Expr::list_from_array([Expr::string("a"), Expr::string("b"), Expr::string("c")]),
+    ]);
+
+    // $ReturnValue = list
+    SymbolExpr::lookup("Global`$ReturnValue").set_to(&result.as_expr());
+}
+
+#[export]
+fn test_kernel_expr_evaluate() {
+    // Evaluate Plus[2, 2]
+    let result = kernel::eval(
+        &NormalExpr::from_slice(&Expr::symbol("System`Plus"), &[
+            Expr::mint(2),
+            Expr::mint(2),
+        ])
+        .into(),
+    );
+
+    let result = MIntExpr::try_from_expr(result)
+        .expect("expected result of evaluating 2 + 2 to be MIntExpr");
+
+    // $ReturnValue = list
+    SymbolExpr::lookup("Global`$ReturnValue").set_to(&result.as_expr());
+}
+
+//======================================
+// Custom DownCode
+//======================================
+
+extern "C" fn eval_downcode(expr: UncountedExpr) -> Expr {
+    let _normal = NormalExpr::try_from_expr_ref(expr.as_expr()).unwrap();
+
+    Expr::string("CUSTOM DOWNCODE")
+}
+
+#[export]
+fn test_kernel_expr_custom_downcode() {
+    let symbol = SymbolExpr::lookup("Global`CustomDownCode");
+
+    symbol.set_downcode(Some(eval_downcode))
+}