Add helper traits/macros to produce FieldResults

Author: mhallin

src/lib.rs

@@ -193,6 +193,7 @@ mod schema;
 mod validation;
 mod executor;
 mod integrations;
+#[macro_use] mod result_ext;
 
 #[cfg(all(test, not(feature="expose-test-schema")))] mod tests;
 #[cfg(feature="expose-test-schema")] pub mod tests;
@@ -216,6 +217,7 @@ pub use executor::{
 pub use validation::RuleError;
 pub use types::scalars::ID;
 pub use schema::model::RootNode;
+pub use result_ext::ResultExt;
 
 pub use schema::meta;
 

src/result_ext.rs

@@ -0,0 +1,63 @@
+use std::fmt;
+use std::result::Result;
+
+/**
+Helper trait to produce `FieldResult`s
+
+`FieldResult` only have strings as errors as that's what's going out
+in the GraphQL response. As such, all errors must be manually
+converted to strings. Importing the `ResultExt` macro and using its
+only method `to_field_err` can help with that:
+
+```rust
+use std::str::FromStr;
+use juniper::{FieldResult, ResultExt};
+
+fn sample_fn(s: &str) -> FieldResult<i64> {
+    i64::from_str(s).to_field_err()
+}
+
+# fn main() { assert_eq!(sample_fn("12"), Ok(12)); }
+```
+
+Alternatively, you can use the `jtry!` macro in all places you'd
+normally use the regular `try!` macro:
+
+```rust
+#[macro_use] extern crate juniper;
+
+use std::str::FromStr;
+
+use juniper::{FieldResult, ResultExt};
+
+fn sample_fn(s: &str) -> FieldResult<i64> {
+    let value = jtry!(i64::from_str(s));
+
+    Ok(value)
+}
+
+# fn main() { assert_eq!(sample_fn("12"), Ok(12)); }
+```
+
+ */
+pub trait ResultExt<T, E: fmt::Display> {
+    /// Convert the error to a string by using it's `Display` implementation
+    fn to_field_err(self) -> Result<T, String>;
+}
+
+impl<T, E: fmt::Display> ResultExt<T, E> for Result<T, E> {
+    fn to_field_err(self) -> Result<T, String> {
+        self.map_err(|e| format!("{}", e))
+    }
+}
+
+/**
+Helper macro to produce `FieldResult`s.
+
+See the documentation for the [`ResultExt`](trait.ResultExt.html)
+trait.
+ */
+#[macro_export]
+macro_rules! jtry {
+    ( $e:expr ) => { try!($crate::ResultExt::to_field_err($e)) }
+}