impl Error for error type; remove `Unknown` error variant

Cargo.toml

@@ -1,7 +1,7 @@
 [package]
 
 name = "secp256k1"
-version = "0.3.1"
+version = "0.3.2"
 authors = [ "Dawid Ciężarkiewicz <dpc@ucore.info>",
             "Andrew Poelstra <apoelstra@wpsoftware.net>" ]
 license = "CC0-1.0"

src/key.rs

@@ -23,7 +23,7 @@ use serialize::{Decoder, Decodable, Encoder, Encodable};
 use serde::{Serialize, Deserialize, Serializer, Deserializer};
 
 use super::{Secp256k1, ContextFlag};
-use super::Error::{self, IncapableContext, InvalidPublicKey, InvalidSecretKey, Unknown};
+use super::Error::{self, IncapableContext, InvalidPublicKey, InvalidSecretKey};
 use constants;
 use ffi;
 
@@ -88,7 +88,7 @@ impl SecretKey {
                      -> Result<(), Error> {
         unsafe {
             if ffi::secp256k1_ec_privkey_tweak_add(secp.ctx, self.as_mut_ptr(), other.as_ptr()) != 1 {
-                Err(Unknown)
+                Err(InvalidSecretKey)
             } else {
                 Ok(())
             }
@@ -180,7 +180,7 @@ impl PublicKey {
                                                   other.as_ptr()) == 1 {
                 Ok(())
             } else {
-                Err(Unknown)
+                Err(InvalidSecretKey)
             }
         }
     }

src/lib.rs

@@ -44,7 +44,7 @@ extern crate libc;
 extern crate rand;
 
 use libc::size_t;
-use std::{fmt, ops, ptr};
+use std::{error, fmt, ops, ptr};
 use rand::Rng;
 
 #[macro_use]
@@ -267,14 +267,28 @@ pub enum Error {
     InvalidSecretKey,
     /// Bad recovery id
     InvalidRecoveryId,
-    /// Boolean-returning function returned the wrong boolean
-    Unknown
 }
 
 // Passthrough Debug to Display, since errors should be user-visible
 impl fmt::Display for Error {
     fn fmt(&self, f: &mut fmt::Formatter) -> Result<(), fmt::Error> {
-        fmt::Debug::fmt(self, f)
+        f.write_str(error::Error::description(self))
+    }
+}
+
+impl error::Error for Error {
+    fn cause(&self) -> Option<&error::Error> { None }
+
+    fn description(&self) -> &str {
+        match *self {
+            Error::IncapableContext => "secp: context does not have sufficient capabilities",
+            Error::IncorrectSignature => "secp: signature failed verification",
+            Error::InvalidMessage => "secp: message was not 32 bytes (do you need to hash?)",
+            Error::InvalidPublicKey => "secp: malformed public key",
+            Error::InvalidSignature => "secp: malformed signature",
+            Error::InvalidSecretKey => "secp: malformed or out-of-range secret key",
+            Error::InvalidRecoveryId => "secp: bad recovery id"
+        }
     }
 }