From f406ec34a354d36052fc889f8efb1e6a51a4aa43 Mon Sep 17 00:00:00 2001
From: Edsko de Vries <edsko@well-typed.com>
Date: Tue, 21 Mar 2023 12:51:32 +0100
Subject: [PATCH] Introduce `marshall_result_to_haskell_var`

---
 haskell-ffi/src/to_haskell.rs | 18 ++++++++++++++++++
 1 file changed, 18 insertions(+)

diff --git a/haskell-ffi/src/to_haskell.rs b/haskell-ffi/src/to_haskell.rs
index da91378..986b326 100644
--- a/haskell-ffi/src/to_haskell.rs
+++ b/haskell-ffi/src/to_haskell.rs
@@ -1,4 +1,5 @@
 use std::{
+    fmt::Display,
     io::{Error, Write},
     marker::PhantomData,
 };
@@ -91,3 +92,20 @@ pub fn marshall_to_haskell_var<Tag, T>(
         Err(e) => panic!("{}", e),
     }
 }
+
+/// Wrapper around `marshall_to_haskell_var` that calls `format` for errors
+pub fn marshall_result_to_haskell_var<Tag, T, E>(
+    res: &Result<T, E>,
+    out: *mut u8,
+    out_len: &mut usize,
+    tag: PhantomData<Tag>,
+) where
+    T: ToHaskell<Tag>,
+    E: Display,
+{
+    let res: Result<&T, String> = match res {
+        Ok(t) => Ok(t),
+        Err(e) => Err(format!("{}", e)),
+    };
+    marshall_to_haskell_var(&res, out, out_len, tag);
+}