Make it an error to specify both before and after for pagination

src/resources/pagination.rs

@@ -1,6 +1,23 @@
+use std::{error, fmt};
+
 use serde;
 use serde_urlencoded;
 
+#[derive(Debug)]
+pub struct Error;
+
+impl fmt::Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
+        write!(f, "{}", (self as &error::Error).description())
+    }
+}
+
+impl error::Error for Error {
+    fn description(&self) -> &str {
+        "`after` and `before` are mutually exclusive"
+    }
+}
+
 #[derive(Deserialize)]
 struct PaginationStruct<T> {
     after: Option<T>,
@@ -14,16 +31,17 @@ pub enum Pagination<T> {
 }
 
 impl<T> PaginationStruct<T> {
-    fn into_enum(self) -> Pagination<T> {
+    fn into_enum(self) -> Result<Pagination<T>, Error> {
         match (self.after, self.before) {
-            (Some(x), _) => Pagination::After(x),
-            (None, Some(x)) => Pagination::Before(x),
-            _ => Pagination::None,
+            (Some(x), None) => Ok(Pagination::After(x)),
+            (None, Some(x)) => Ok(Pagination::Before(x)),
+            (None, None) => Ok(Pagination::None),
+            _ => Err(Error)
         }
     }
 }
 
-pub fn from_str<'a, T: serde::Deserialize<'a>>(s: &'a str) -> Result<Pagination<T>, serde::de::value::Error> {
-    let pagination: PaginationStruct<T> = serde_urlencoded::from_str(s)?;
-    Ok(pagination.into_enum())
+pub fn from_str<'a, T: serde::Deserialize<'a>>(s: &'a str) -> Result<Pagination<T>, Error> {
+    let pagination: PaginationStruct<T> = serde_urlencoded::from_str(s).map_err(|_| Error)?; // TODO Proper error reporting
+    Ok(pagination.into_enum()?)
 }