Make error type explicit

Signed-off-by: Matthias Beyer <mail@beyermatthias.de>

10 files changed, 151 insertions, 187 deletions

diff --git a/examples/option.rs b/examples/option.rs
index 937733a..2a30eed 100644
--- a/examples/option.rs
+++ b/examples/option.rs
@@ -1,15 +1,11 @@
 use interactive_object_builder::*;
 use interactive_object_builder::backend::dialoguer::DialoguerBackend;
 
-fn main() -> Result<(), ()> {
-    let v = Option::<bool>::builder(DialoguerBackend).build_collection("optional yes/no")
-        .map_err(|_| ())?;
-
+fn main() -> Result<()> {
+    let v = Option::<bool>::builder(DialoguerBackend).build_collection("optional yes/no")?;
     println!("{:?}", v);
 
-    let v = Option::<u64>::builder(DialoguerBackend).build_collection("optional u64")
-        .map_err(|_| ())?;
-
+    let v = Option::<u64>::builder(DialoguerBackend).build_collection("optional u64")?;
     println!("{:?}", v);
     Ok(())
 }
diff --git a/examples/simple.rs b/examples/simple.rs
index 46dedfd..bd406c4 100644
--- a/examples/simple.rs
+++ b/examples/simple.rs
@@ -1,9 +1,8 @@
 use interactive_object_builder::*;
 use interactive_object_builder::backend::dialoguer::DialoguerBackend;
 
-fn main() -> Result<(), ()> {
-    let b = bool::builder(DialoguerBackend).build_value("Success?")
-        .map_err(|_| ())?;
+fn main() -> Result<()> {
+    let b = bool::builder(DialoguerBackend).build_value("Success?")?;
 
     if b {
         println!("Success");
diff --git a/examples/vec.rs b/examples/vec.rs
index 6aa10c4..6adeacc 100644
--- a/examples/vec.rs
+++ b/examples/vec.rs
@@ -1,9 +1,8 @@
 use interactive_object_builder::*;
 use interactive_object_builder::backend::dialoguer::DialoguerBackend;
 
-fn main() -> Result<(), ()> {
-    let v = Vec::<bool>::builder(DialoguerBackend).build_collection("yes/no flag")
-        .map_err(|_| ())?;
+fn main() -> Result<()> {
+    let v = Vec::<bool>::builder(DialoguerBackend).build_collection("yes/no flag")?;
 
     println!("{:?}", v);
     Ok(())
diff --git a/src/backend/dialoguer.rs b/src/backend/dialoguer.rs
index 5bdebd6..ec1f5b8 100644
--- a/src/backend/dialoguer.rs
+++ b/src/backend/dialoguer.rs
@@ -1,3 +1,4 @@
+use crate::error::Result;
 use crate::Backend;
 use crate::BuildableValue;
 use crate::CollectionBuilder;
@@ -5,90 +6,82 @@ use crate::ValueBuilder;
 
 pub struct DialoguerBackend;
 
-#[derive(Debug, thiserror::Error)]
-pub enum DialoguerBackendError {
-    #[error("")]
-    Io(#[from] std::io::Error),
-}
-
 impl Backend for DialoguerBackend {
-    type Error = DialoguerBackendError;
-
-    fn bool_builder(&self) -> Box<dyn ValueBuilder<Output = bool, Error = Self::Error>> {
+    fn bool_builder(&self) -> Box<dyn ValueBuilder<Output = bool>> {
         Box::new(DialoguerValueBuilder::<bool>(std::marker::PhantomData))
     }
 
-    fn u8_builder(&self)     -> Box<dyn ValueBuilder<Output = u8, Error = Self::Error>> {
+    fn u8_builder(&self)     -> Box<dyn ValueBuilder<Output = u8>> {
         Box::new(DialoguerValueBuilder::<u8>(std::marker::PhantomData))
     }
 
-    fn u16_builder(&self)    -> Box<dyn ValueBuilder<Output = u16, Error = Self::Error>> {
+    fn u16_builder(&self)    -> Box<dyn ValueBuilder<Output = u16>> {
         Box::new(DialoguerValueBuilder::<u16>(std::marker::PhantomData))
     }
 
-    fn u32_builder(&self)    -> Box<dyn ValueBuilder<Output = u32, Error = Self::Error>> {
+    fn u32_builder(&self)    -> Box<dyn ValueBuilder<Output = u32>> {
         Box::new(DialoguerValueBuilder::<u32>(std::marker::PhantomData))
     }
 
-    fn u64_builder(&self)    -> Box<dyn ValueBuilder<Output = u64, Error = Self::Error>> {
+    fn u64_builder(&self)    -> Box<dyn ValueBuilder<Output = u64>> {
         Box::new(DialoguerValueBuilder::<u64>(std::marker::PhantomData))
     }
 
-    fn u128_builder(&self)   -> Box<dyn ValueBuilder<Output = u128, Error = Self::Error>> {
+    fn u128_builder(&self)   -> Box<dyn ValueBuilder<Output = u128>> {
         Box::new(DialoguerValueBuilder::<u128>(std::marker::PhantomData))
     }
 
-    fn i8_builder(&self)     -> Box<dyn ValueBuilder<Output = i8, Error = Self::Error>> {
+    fn i8_builder(&self)     -> Box<dyn ValueBuilder<Output = i8>> {
         Box::new(DialoguerValueBuilder::<i8>(std::marker::PhantomData))
     }
 
-    fn i16_builder(&self)    -> Box<dyn ValueBuilder<Output = i16, Error = Self::Error>> {
+    fn i16_builder(&self)    -> Box<dyn ValueBuilder<Output = i16>> {
         Box::new(DialoguerValueBuilder::<i16>(std::marker::PhantomData))
     }
 
-    fn i32_builder(&self)    -> Box<dyn ValueBuilder<Output = i32, Error = Self::Error>> {
+    fn i32_builder(&self)    -> Box<dyn ValueBuilder<Output = i32>> {
         Box::new(DialoguerValueBuilder::<i32>(std::marker::PhantomData))
     }
 
-    fn i64_builder(&self)    -> Box<dyn ValueBuilder<Output = i64, Error = Self::Error>> {
+    fn i64_builder(&self)    -> Box<dyn ValueBuilder<Output = i64>> {
         Box::new(DialoguerValueBuilder::<i64>(std::marker::PhantomData))
     }
 
-    fn i128_builder(&self)   -> Box<dyn ValueBuilder<Output = i128, Error = Self::Error>> {
+    fn i128_builder(&self)   -> Box<dyn ValueBuilder<Output = i128>> {
         Box::new(DialoguerValueBuilder::<i128>(std::marker::PhantomData))
     }
 
-    fn usize_builder(&self)  -> Box<dyn ValueBuilder<Output = usize, Error = Self::Error>> {
+    fn usize_builder(&self)  -> Box<dyn ValueBuilder<Output = usize>> {
         Box::new(DialoguerValueBuilder::<usize>(std::marker::PhantomData))
     }
 
-    fn isize_builder(&self)  -> Box<dyn ValueBuilder<Output = isize, Error = Self::Error>> {
+    fn isize_builder(&self)  -> Box<dyn ValueBuilder<Output = isize>> {
         Box::new(DialoguerValueBuilder::<isize>(std::marker::PhantomData))
     }
 
-    fn f32_builder(&self)    -> Box<dyn ValueBuilder<Output = f32, Error = Self::Error>> {
+    fn f32_builder(&self)    -> Box<dyn ValueBuilder<Output = f32>> {
         Box::new(DialoguerValueBuilder::<f32>(std::marker::PhantomData))
     }
 
-    fn f64_builder(&self)    -> Box<dyn ValueBuilder<Output = f64, Error = Self::Error>> {
+    fn f64_builder(&self)    -> Box<dyn ValueBuilder<Output = f64>> {
         Box::new(DialoguerValueBuilder::<f64>(std::marker::PhantomData))
     }
 
-    fn char_builder(&self)   -> Box<dyn ValueBuilder<Output = char, Error = Self::Error>> {
+    fn char_builder(&self)   -> Box<dyn ValueBuilder<Output = char>> {
         Box::new(DialoguerValueBuilder::<char>(std::marker::PhantomData))
     }
 
-    fn string_builder(&self) -> Box<dyn ValueBuilder<Output = String, Error = Self::Error>> {
+    fn string_builder(&self) -> Box<dyn ValueBuilder<Output = String>> {
         Box::new(DialoguerValueBuilder::<String>(std::marker::PhantomData))
     }
 
-    fn vec_builder<T>(&self) -> Box<dyn CollectionBuilder<Output = Vec<T>, Error = Self::Error>>
+    fn vec_builder<T>(&self) -> Box<dyn CollectionBuilder<Output = Vec<T>>>
         where T: 'static + BuildableValue
     {
         Box::new(DialoguerVecBuilder::<T>(std::marker::PhantomData))
     }
 
-    fn option_builder<T: 'static + BuildableValue>(&self) -> Box<dyn CollectionBuilder<Output = Option<T>, Error = Self::Error>> {
+    fn option_builder<T: 'static + BuildableValue>(&self) -> Box<dyn CollectionBuilder<Output = Option<T>>> {
         Box::new(DialoguerOptionValueBuilder::<T>(std::marker::PhantomData))
     }
 }
@@ -99,22 +92,20 @@ impl<T> ValueBuilder for DialoguerValueBuilder<T>
           T::Err: std::fmt::Display + std::fmt::Debug,
 {
     type Output = T;
-    type Error = DialoguerBackendError;
 
-    fn build_value(&self, question: &str) -> Result<Self::Output, Self::Error> {
+    fn build_value(&self, question: &str) -> Result<Self::Output> {
         dialoguer::Input::<T>::new()
             .with_prompt(question)
             .interact_text()
-            .map_err(DialoguerBackendError::from)
+            .map_err(crate::error::IOBError::from)
     }
 }
 
 pub struct DialoguerOptionValueBuilder<T: BuildableValue>(std::marker::PhantomData<T>);
 impl<T: BuildableValue> CollectionBuilder for DialoguerOptionValueBuilder<T> {
     type Output = Option<T>;
-    type Error = DialoguerBackendError;
 
-    fn build_collection(&self, value_desc: &str) -> Result<Self::Output, Self::Error> {
+    fn build_collection(&self, value_desc: &str) -> Result<Self::Output> {
         let q = format!("Do you want to provide a Value for {}", value_desc);
         let b = dialoguer::Input::<bool>::new()
             .with_prompt(q)
@@ -136,9 +127,8 @@ impl<T> CollectionBuilder for DialoguerVecBuilder<T>
     where T: 'static + BuildableValue
 {
     type Output = Vec<T>;
-    type Error = DialoguerBackendError;
 
-    fn build_collection(&self, value_desc: &str) -> Result<Self::Output, Self::Error> {
+    fn build_collection(&self, value_desc: &str) -> Result<Self::Output> {
         let mut buf = vec![];
 
         loop {
diff --git a/src/backend/mod.rs b/src/backend/mod.rs
index 3f72d66..5884726 100644
--- a/src/backend/mod.rs
+++ b/src/backend/mod.rs
@@ -6,29 +6,27 @@ use crate::ValueBuilder;
 
 /// A backend can be used to get builders for specific types
 pub trait Backend {
-    type Error: Sized;
+    fn bool_builder(&self)   -> Box<dyn ValueBuilder<Output = bool>>;
+    fn u8_builder(&self)     -> Box<dyn ValueBuilder<Output = u8>>;
+    fn u16_builder(&self)    -> Box<dyn ValueBuilder<Output = u16>>;
+    fn u32_builder(&self)    -> Box<dyn ValueBuilder<Output = u32>>;
+    fn u64_builder(&self)    -> Box<dyn ValueBuilder<Output = u64>>;
+    fn u128_builder(&self)   -> Box<dyn ValueBuilder<Output = u128>>;
+    fn i8_builder(&self)     -> Box<dyn ValueBuilder<Output = i8>>;
+    fn i16_builder(&self)    -> Box<dyn ValueBuilder<Output = i16>>;
+    fn i32_builder(&self)    -> Box<dyn ValueBuilder<Output = i32>>;
+    fn i64_builder(&self)    -> Box<dyn ValueBuilder<Output = i64>>;
+    fn i128_builder(&self)   -> Box<dyn ValueBuilder<Output = i128>>;
+    fn usize_builder(&self)  -> Box<dyn ValueBuilder<Output = usize>>;
+    fn isize_builder(&self)  -> Box<dyn ValueBuilder<Output = isize>>;
+    fn f32_builder(&self)    -> Box<dyn ValueBuilder<Output = f32>>;
+    fn f64_builder(&self)    -> Box<dyn ValueBuilder<Output = f64>>;
+    fn char_builder(&self)   -> Box<dyn ValueBuilder<Output = char>>;
+    fn string_builder(&self) -> Box<dyn ValueBuilder<Output = String>>;
 
-    fn bool_builder(&self)   -> Box<dyn ValueBuilder<Output = bool, Error = Self::Error>>;
-    fn u8_builder(&self)     -> Box<dyn ValueBuilder<Output = u8, Error = Self::Error>>;
-    fn u16_builder(&self)    -> Box<dyn ValueBuilder<Output = u16, Error = Self::Error>>;
-    fn u32_builder(&self)    -> Box<dyn ValueBuilder<Output = u32, Error = Self::Error>>;
-    fn u64_builder(&self)    -> Box<dyn ValueBuilder<Output = u64, Error = Self::Error>>;
-    fn u128_builder(&self)   -> Box<dyn ValueBuilder<Output = u128, Error = Self::Error>>;
-    fn i8_builder(&self)     -> Box<dyn ValueBuilder<Output = i8, Error = Self::Error>>;
-    fn i16_builder(&self)    -> Box<dyn ValueBuilder<Output = i16, Error = Self::Error>>;
-    fn i32_builder(&self)    -> Box<dyn ValueBuilder<Output = i32, Error = Self::Error>>;
-    fn i64_builder(&self)    -> Box<dyn ValueBuilder<Output = i64, Error = Self::Error>>;
-    fn i128_builder(&self)   -> Box<dyn ValueBuilder<Output = i128, Error = Self::Error>>;
-    fn usize_builder(&self)  -> Box<dyn ValueBuilder<Output = usize, Error = Self::Error>>;
-    fn isize_builder(&self)  -> Box<dyn ValueBuilder<Output = isize, Error = Self::Error>>;
-    fn f32_builder(&self)    -> Box<dyn ValueBuilder<Output = f32, Error = Self::Error>>;
-    fn f64_builder(&self)    -> Box<dyn ValueBuilder<Output = f64, Error = Self::Error>>;
-    fn char_builder(&self)   -> Box<dyn ValueBuilder<Output = char, Error = Self::Error>>;
-    fn string_builder(&self) -> Box<dyn ValueBuilder<Output = String, Error = Self::Error>>;
+    fn option_builder<T: 'static + BuildableValue>(&self) -> Box<dyn CollectionBuilder<Output = Option<T>>>;
 
-    fn option_builder<T: 'static + BuildableValue>(&self) -> Box<dyn CollectionBuilder<Output = Option<T>, Error = Self::Error>>;
-
-    fn vec_builder<T>(&self) -> Box<dyn CollectionBuilder<Output = Vec<T>, Error = Self::Error>>
+    fn vec_builder<T>(&self) -> Box<dyn CollectionBuilder<Output = Vec<T>>>
         where T: 'static + BuildableValue;
 }
 
diff --git a/src/buildable.rs b/src/buildable.rs
index 37bfc3c..54e7bbe 100644
--- a/src/buildable.rs
+++ b/src/buildable.rs
@@ -4,160 +4,141 @@ use crate::ValueBuilder;
 
 // A value that can be built interactively, using a Backend
 pub trait BuildableValue: Sized {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = Self, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
-        ;
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = Self>>
+        where B: Backend;
 }
 
 impl BuildableValue for bool {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = Self, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = Self>>
+        where B: Backend
     {
         backend.bool_builder()
     }
 }
 
 impl BuildableValue for u8 {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = u8, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = u8>>
+        where B: Backend
     {
         backend.u8_builder()
     }
 }
 
 impl BuildableValue for u16 {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = u16, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = u16>>
+        where B: Backend
     {
         backend.u16_builder()
     }
 }
 
 impl BuildableValue for u32 {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = u32, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = u32>>
+        where B: Backend
     {
         backend.u32_builder()
     }
 }
 
 impl BuildableValue for u64 {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = u64, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = u64>>
+        where B: Backend
     {
         backend.u64_builder()
     }
 }
 
 impl BuildableValue for u128 {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = u128, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = u128>>
+        where B: Backend
     {
         backend.u128_builder()
     }
 }
 
 impl BuildableValue for i8 {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = i8, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = i8>>
+        where B: Backend
     {
         backend.i8_builder()
     }
 }
 
 impl BuildableValue for i16 {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = i16, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = i16>>
+        where B: Backend
     {
         backend.i16_builder()
     }
 }
 
 impl BuildableValue for i32 {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = i32, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = i32>>
+        where B: Backend
     {
         backend.i32_builder()
     }
 }
 
 impl BuildableValue for i64 {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = i64, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = i64>>
+        where B: Backend
     {
         backend.i64_builder()
     }
 }
 
 impl BuildableValue for i128 {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = i128, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = i128>>
+        where B: Backend
     {
         backend.i128_builder()
     }
 }
 
 impl BuildableValue for usize {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = usize, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = usize>>
+        where B: Backend
     {
         backend.usize_builder()
     }
 }
 
 impl BuildableValue for isize {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = isize, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = isize>>
+        where B: Backend
     {
         backend.isize_builder()
     }
 }
 
 impl BuildableValue for f32 {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = f32, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = f32>>
+        where B: Backend
     {
         backend.f32_builder()
     }
 }
 
 impl BuildableValue for f64 {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = f64, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = f64>>
+        where B: Backend
     {
         backend.f64_builder()
     }
 }
 
 impl BuildableValue for char {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = char, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = char>>
+        where B: Backend
     {
         backend.char_builder()
     }
 }
 
 impl BuildableValue for String {
-    fn builder<E, B>(backend: B) -> Box<dyn ValueBuilder<Output = String, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn ValueBuilder<Output = String>>
+        where B: Backend
     {
         backend.string_builder()
     }
@@ -165,29 +146,25 @@ impl BuildableValue for String {
 
 
 pub trait BuildableCollection {
-    fn builder<E, B>(backend: B) -> Box<dyn CollectionBuilder<Output = Self, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
-        ;
+    fn builder<B>(backend: B) -> Box<dyn CollectionBuilder<Output = Self>>
+        where B: Backend;
 }
 
 impl<T> BuildableCollection for Vec<T>
-    where T: 'static + BuildableValue
+    where T: 'static + BuildableValue,
 {
-    fn builder<E, B>(backend: B) -> Box<dyn CollectionBuilder<Output = Self, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn CollectionBuilder<Output = Self>>
+        where B: Backend
     {
         backend.vec_builder::<T>()
     }
 }
 
 impl<T> BuildableCollection for Option<T>
-    where T: 'static + BuildableValue + Sized
+    where T: 'static + BuildableValue + Sized,
 {
-    fn builder<E, B>(backend: B) -> Box<dyn CollectionBuilder<Output = Option<T>, Error = E>>
-        where E: Sized,
-              B: Backend<Error = E>
+    fn builder<B>(backend: B) -> Box<dyn CollectionBuilder<Output = Option<T>>>
+        where B: Backend
     {
         backend.option_builder()
     }
diff --git a/src/collection_builder.rs b/src/collection_builder.rs
index 70c1ae3..ee49896 100644
--- a/src/collection_builder.rs
+++ b/src/collection_builder.rs
@@ -1,10 +1,11 @@
+use crate::error::Result;
+
 /// A collection type that can be built interactively
 ///
 /// That is, for example, a Vec<_>, a HashMap<_, _> or a struct
 pub trait CollectionBuilder {
     type Output: Sized;
-    type Error: Sized;
 
-    fn build_collection(&self, value_desc: &str) -> Result<Self::Output, Self::Error>;
+    fn build_collection(&self, value_desc: &str) -> Result<Self::Output>;
 }
 
diff --git a/src/error.rs b/src/error.rs
new file mode 100644
index 0000000..bfcf175
--- /dev/null
+++ b/src/error.rs
@@ -0,0 +1,7 @@
+#[derive(Debug, thiserror::Error)]
+pub enum IOBError {
+    #[error("{}", .0)]
+    Io(#[from] std::io::Error),
+}
+
+pub type Result<T> = std::result::Result<T, IOBError>;
diff --git a/src/lib.rs b/src/lib.rs
index f7ef93b..9a89f37 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -7,6 +7,9 @@ pub use crate::buildable::*;
 pub mod collection_builder;
 pub use crate::collection_builder::*;
 
+pub mod error;
+pub use crate::error::*;
+
 pub mod value_builder;
 pub use crate::value_builder::*;
 
@@ -17,65 +20,63 @@ mod dummy_bool_test {
 
     struct DummyBackend;
     impl Backend for DummyBackend {
-        type Error = ();
-
-        fn bool_builder(&self) -> Box<dyn ValueBuilder<Output = bool, Error = Self::Error>> {
+        fn bool_builder(&self) -> Box<dyn ValueBuilder<Output = bool>> {
             Box::new(BoolValBuilder)
         }
-        fn u8_builder(&self)     -> Box<dyn ValueBuilder<Output = u8, Error = Self::Error>> {
+        fn u8_builder(&self)     -> Box<dyn ValueBuilder<Output = u8>> {
             unimplemented!()
         }
-        fn u16_builder(&self)    -> Box<dyn ValueBuilder<Output = u16, Error = Self::Error>> {
+        fn u16_builder(&self)    -> Box<dyn ValueBuilder<Output = u16>> {
             unimplemented!()
         }
-        fn u32_builder(&self)    -> Box<dyn ValueBuilder<Output = u32, Error = Self::Error>> {
+        fn u32_builder(&self)    -> Box<dyn ValueBuilder<Output = u32>> {
             unimplemented!()
         }
-        fn u64_builder(&self)    -> Box<dyn ValueBuilder<Output = u64, Error = Self::Error>> {
+        fn u64_builder(&self)    -> Box<dyn ValueBuilder<Output = u64>> {
             unimplemented!()
         }
-        fn u128_builder(&self)   -> Box<dyn ValueBuilder<Output = u128, Error = Self::Error>> {
+        fn u128_builder(&self)   -> Box<dyn ValueBuilder<Output = u128>> {
             unimplemented!()
         }
-        fn i8_builder(&self)     -> Box<dyn ValueBuilder<Output = i8, Error = Self::Error>> {
+        fn i8_builder(&self)     -> Box<dyn ValueBuilder<Output = i8>> {
             unimplemented!()
         }
-        fn i16_builder(&self)    -> Box<dyn ValueBuilder<Output = i16, Error = Self::Error>> {
+        fn i16_builder(&self)    -> Box<dyn ValueBuilder<Output = i16>> {
             unimplemented!()
         }
-        fn i32_builder(&self)    -> Box<dyn ValueBuilder<Output = i32, Error = Self::Error>> {
+        fn i32_builder(&self)    -> Box<dyn ValueBuilder<Output = i32>> {
             unimplemented!()
         }
-        fn i64_builder(&self)    -> Box<dyn ValueBuilder<Output = i64, Error = Self::Error>> {
+        fn i64_builder(&self)    -> Box<dyn ValueBuilder<Output = i64>> {
             unimplemented!()
         }
-        fn i128_builder(&self)   -> Box<dyn ValueBuilder<Output = i128, Error = Self::Error>> {
+        fn i128_builder(&self)   -> Box<dyn ValueBuilder<Output = i128>> {
             unimplemented!()
         }
-        fn usize_builder(&self)  -> Box<dyn ValueBuilder<Output = usize, Error = Self::Error>> {
+        fn usize_builder(&self)  -> Box<dyn ValueBuilder<Output = usize>> {
             unimplemented!()
         }
-        fn isize_builder(&self)  -> Box<dyn ValueBuilder<Output = isize, Error = Self::Error>> {
+        fn isize_builder(&self)  -> Box<dyn ValueBuilder<Output = isize>> {
             unimplemented!()
         }
-        fn f32_builder(&self)    -> Box<dyn ValueBuilder<Output = f32, Error = Self::Error>> {
+        fn f32_builder(&self)    -> Box<dyn ValueBuilder<Output = f32>> {
             unimplemented!()
         }
-        fn f64_builder(&self)    -> Box<dyn ValueBuilder<Output = f64, Error = Self::Error>> {
+        fn f64_builder(&self)    -> Box<dyn ValueBuilder<Output = f64>> {
             unimplemented!()
         }
-        fn char_builder(&self)   -> Box<dyn ValueBuilder<Output = char, Error = Self::Error>> {
+        fn char_builder(&self)   -> Box<dyn ValueBuilder<Output = char>> {
             unimplemented!()
         }
-        fn string_builder(&self) -> Box<dyn ValueBuilder<Output = String, Error = Self::Error>> {
+        fn string_builder(&self) -> Box<dyn ValueBuilder<Output = String>> {
             unimplemented!()
         }
 
-        fn option_builder<T: 'static + BuildableValue>(&self) -> Box<dyn CollectionBuilder<Output = Option<T>, Error = Self::Error>> {
+        fn option_builder<T: 'static + BuildableValue>(&self) -> Box<dyn CollectionBuilder<Output = Option<T>>> {
             unimplemented!()
         }
 
-        fn vec_builder<T>(&self) -> Box<dyn CollectionBuilder<Output = Vec<T>, Error = Self::Error>>
+        fn vec_builder<T>(&self) -> Box<dyn CollectionBuilder<Output = Vec<T>>>
             where T: 'static + BuildableValue
         {
             unimplemented!()
@@ -85,9 +86,8 @@ mod dummy_bool_test {
     struct BoolValBuilder;
     impl ValueBuilder for BoolValBuilder {
         type Output = bool;
-        type Error = ();
 
-        fn build_value(&self, _question: &str) -> Result<Self::Output, Self::Error> {
+        fn build_value(&self, _question: &str) -> Result<Self::Output> {
             Ok(true)
         }
     }
@@ -106,63 +106,61 @@ mod vec_test {
 
     struct DummyBackend;
     impl Backend for DummyBackend {
-        type Error = ();
-
-        fn bool_builder(&self) -> Box<dyn ValueBuilder<Output = bool, Error = Self::Error>> {
+        fn bool_builder(&self) -> Box<dyn ValueBuilder<Output = bool>> {
             Box::new(BoolValBuilder)
         }
-        fn u8_builder(&self)     -> Box<dyn ValueBuilder<Output = u8, Error = Self::Error>> {
+        fn u8_builder(&self)     -> Box<dyn ValueBuilder<Output = u8>> {
             unimplemented!()
         }
-        fn u16_builder(&self)    -> Box<dyn ValueBuilder<Output = u16, Error = Self::Error>> {
+        fn u16_builder(&self)    -> Box<dyn ValueBuilder<Output = u16>> {
             unimplemented!()
         }
-        fn u32_builder(&self)    -> Box<dyn ValueBuilder<Output = u32, Error = Self::Error>> {
+        fn u32_builder(&self)    -> Box<dyn ValueBuilder<Output = u32>> {
             unimplemented!()
         }
-        fn u64_builder(&self)    -> Box<dyn ValueBuilder<Output = u64, Error = Self::Error>> {
+        fn u64_builder(&self)    -> Box<dyn ValueBuilder<Output = u64>> {
             unimplemented!()
         }
-        fn u128_builder(&self)   -> Box<dyn ValueBuilder<Output = u128, Error = Self::Error>> {
+        fn u128_builder(&self)   -> Box<dyn ValueBuilder<Output = u128>> {
             unimplemented!()
         }
-        fn i8_builder(&self)     -> Box<dyn ValueBuilder<Output = i8, Error = Self::Error>> {
+        fn i8_builder(&self)     -> Box<dyn ValueBuilder<Output = i8>> {
             unimplemented!()
         }
-        fn i16_builder(&self)    -> Box<dyn ValueBuilder<Output = i16, Error = Self::Error>> {
+        fn i16_builder(&self)    -> Box<dyn ValueBuilder<Output = i16>> {
             unimplemented!()
         }
-        fn i32_builder(&self)    -> Box<dyn ValueBuilder<Output = i32, Error = Self::Error>> {
+        fn i32_builder(&self)    -> Box<dyn ValueBuilder<Output = i32>> {
             unimplemented!()
         }
-        fn i64_builder(&self)    -> Box<dyn ValueBuilder<Output = i64, Error = Self::Error>> {
+        fn i64_builder(&self)    -> Box<dyn ValueBuilder<Output = i64>> {
             unimplemented!()
         }
-        fn i128_builder(&self)   -> Box<dyn ValueBuilder<Output = i128, Error = Self::Error>> {
+        fn i128_builder(&self)   -> Box<dyn ValueBuilder<Output = i128>> {
             unimplemented!()
         }
-        fn usize_builder(&self)  -> Box<dyn ValueBuilder<Output = usize, Error = Self::Error>> {
+        fn usize_builder(&self)  -> Box<dyn ValueBuilder<Output = usize>> {
             unimplemented!()
         }
-        fn isize_builder(&self)  -> Box<dyn ValueBuilder<Output = isize, Error = Self::Error>> {
+        fn isize_builder(&self)  -> Box<dyn ValueBuilder<Output = isize>> {
             unimplemented!()
         }
-        fn f32_builder(&self)    -> Box<dyn ValueBuilder<Output = f32, Error = Self::Error>> {
+        fn f32_builder(&self)    -> Box<dyn ValueBuilder<Output = f32>> {
             unimplemented!()
         }
-        fn f64_builder(&self)    -> Box<dyn ValueBuilder<Output = f64, Error = Self::Error>> {
+        fn f64_builder(&self)    -> Box<dyn ValueBuilder<Output = f64>> {
             unimplemented!()
         }
-        fn char_builder(&self)   -> Box<dyn ValueBuilder<Output = char, Error = Self::Error>> {
+        fn char_builder(&self)   -> Box<dyn ValueBuilder<Output = char>> {
             unimplemented!()
         }
-        fn string_builder(&self) -> Box<dyn ValueBuilder<Output = String, Error = Self::Error>> {
+        fn string_builder(&self) -> Box<dyn ValueBuilder<Output = String>> {
             unimplemented!()
         }
-        fn option_builder<T: 'static + BuildableValue>(&self) -> Box<dyn CollectionBuilder<Output = Option<T>, Error = Self::Error>> {
+        fn option_builder<T: 'static + BuildableValue>(&self) -> Box<dyn CollectionBuilder<Output = Option<T>>> {
             unimplemented!()
         }
-        fn vec_builder<T>(&self) -> Box<dyn CollectionBuilder<Output = Vec<T>, Error = Self::Error>>
+        fn vec_builder<T>(&self) -> Box<dyn CollectionBuilder<Output = Vec<T>>>
             where T: 'static + BuildableValue
         {
             Box::new(VecBuilder::<T>(std::marker::PhantomData))
@@ -172,9 +170,8 @@ mod vec_test {
     struct BoolValBuilder;
     impl ValueBuilder for BoolValBuilder {
         type Output = bool;
-        type Error = ();
 
-        fn build_value(&self, _question: &str) -> Result<Self::Output, Self::Error> {
+        fn build_value(&self, _question: &str) -> Result<Self::Output> {
             Ok(true)
         }
     }
@@ -182,9 +179,8 @@ mod vec_test {
     pub struct VecBuilder<T: 'static + BuildableValue>(std::marker::PhantomData<T>);
     impl<T: BuildableValue> CollectionBuilder for VecBuilder<T> {
         type Output = Vec<T>;
-        type Error = ();
 
-        fn build_collection(&self, value_desc: &str) -> Result<Self::Output, Self::Error> {
+        fn build_collection(&self, value_desc: &str) -> Result<Self::Output> {
             let mut buf = vec![];
 
             loop {
diff --git a/src/value_builder.rs b/src/value_builder.rs
index 5418ee6..d7e045b 100644
--- a/src/value_builder.rs
+++ b/src/value_builder.rs
@@ -1,8 +1,9 @@
+use crate::error::Result;
+
 /// A thing that can build a value by interactively asking the user a question
 pub trait ValueBuilder {
     type Output: Sized;
-    type Error: Sized;
 
-    fn build_value(&self, question: &str) -> Result<Self::Output, Self::Error>;
+    fn build_value(&self, question: &str) -> Result<Self::Output>;
 }