diff --git a/crates/generate/src/errors.rs b/crates/generate/src/errors.rs
new file mode 100644
index 0000000000..d1b4bbfbc3
--- /dev/null
+++ b/crates/generate/src/errors.rs
@@ -0,0 +1,45 @@
+use crate::names::Names;
+use heck::SnakeCase;
+use proc_macro2::TokenStream;
+use quote::{format_ident, quote};
+use std::collections::HashSet;
+use witx::{Document, TypeRef};
+
+/// The context struct needs to implement a trait for converting memory and value errors into the
+/// witx doc's error types.
+pub fn define_error_trait(names: &Names, doc: &Document) -> TokenStream {
+    // All non-anonymous first return types are used to pass errors.
+    let error_typenames = doc
+        .modules()
+        .flat_map(|m| {
+            m.funcs()
+                .filter_map(|f| {
+                    f.results.get(0).and_then(|r| match &r.tref {
+                        TypeRef::Name(nt) => Some(nt.name.clone()),
+                        _ => None,
+                    })
+                })
+                .collect::<HashSet<witx::Id>>()
+        })
+        .collect::<HashSet<witx::Id>>();
+
+    let methods = error_typenames.iter().map(|typename| {
+        let tname = names.type_(typename);
+        let methodfragment = typename.as_str().to_snake_case();
+        let success = format_ident!("success_to_{}", methodfragment);
+        let memory_error = format_ident!("memory_error_to_{}", methodfragment);
+        let value_error = format_ident!("value_error_to_{}", methodfragment);
+
+        quote! {
+            fn #success(&mut self) -> #tname;
+            fn #memory_error(&mut self, err: ::memory::MemoryError) -> #tname;
+            fn #value_error(&mut self, err: ::memory::GuestValueError) -> #tname;
+        }
+    });
+
+    quote!(
+        pub trait WitxErrorConversion {
+            #(#methods)*
+        }
+    )
+}
diff --git a/crates/generate/src/funcs.rs b/crates/generate/src/funcs.rs
index 3f856b863a..bfe720f2d9 100644
--- a/crates/generate/src/funcs.rs
+++ b/crates/generate/src/funcs.rs
@@ -3,44 +3,45 @@ use quote::quote;
 
 use crate::names::Names;
 
-// FIXME need to template what arguments are required to an import function - some context
+// FIXME need to template what argument is required to an import function - some context
 // struct (e.g. WasiCtx) should be provided at the invocation of the `gen` proc macro.
-// Rather than pass in memory as a `&mut [u8]` as today, require a `GuestMemory<'a>` be
-// passed in.
+//
+// Additionally - need to template how to transform GuestValueError and MemoryError into
+// the error type returned! From impl is a good start, but what if we want to log
+// a more informative error? Maybe the conversion should be a (generated, for each by-value first
+// return type) trait impled by WasiCtx?
 pub fn define_func(names: &Names, func: &witx::InterfaceFunc) -> TokenStream {
     let ident = names.func(&func.name);
-    let mut args = quote!(wasi_ctx: &mut WasiCtx, memory: GuestMemory,);
 
     let arg_signature = |param: &witx::InterfaceFuncParam| -> TokenStream {
         let name = names.func_param(&param.name);
         match param.tref.type_().passed_by() {
             witx::TypePassedBy::Value(atom) => {
                 let atom = names.atom_type(atom);
-                quote!(#name: #atom,)
+                quote!(#name: #atom)
             }
             witx::TypePassedBy::Pointer => {
                 let atom = names.atom_type(witx::AtomType::I32);
-                quote!(#name: #atom,)
+                quote!(#name: #atom)
             }
             witx::TypePassedBy::PointerLengthPair => {
                 let atom = names.atom_type(witx::AtomType::I32);
                 let len_name = names.func_len_param(&param.name);
-                quote!(#name: #atom, #len_name, #atom,)
+                quote!(#name: #atom, #len_name: #atom)
             }
         }
     };
 
-    for param in func.params.iter() {
-        args.extend(arg_signature(param));
-    }
-
-    if let Some(arg_results) = func.results.get(1..) {
-        for result in arg_results {
-            args.extend(arg_signature(result))
-        }
-    }
-
-    let ret = if let Some(first_result) = func.results.get(0) {
+    let params = func
+        .params
+        .iter()
+        .chain(func.results.iter().skip(1))
+        .map(arg_signature);
+    let abi_args = quote!(
+            wasi_ctx: &mut WasiCtx, memory: GuestMemory,
+            #(#params),*
+    );
+    let abi_ret = if let Some(first_result) = func.results.get(0) {
         match first_result.tref.type_().passed_by() {
             witx::TypePassedBy::Value(atom) => names.atom_type(atom),
             _ => unreachable!("first result should always be passed by value"),
@@ -51,5 +52,5 @@ pub fn define_func(names: &Names, func: &witx::InterfaceFunc) -> TokenStream {
         quote!(())
     };
 
-    quote!(pub fn #ident(#args) -> #ret { unimplemented!() })
+    quote!(pub fn #ident(#abi_args) -> #abi_ret { unimplemented!() })
 }
diff --git a/crates/generate/src/lib.rs b/crates/generate/src/lib.rs
index acf60e1401..830e92e900 100644
--- a/crates/generate/src/lib.rs
+++ b/crates/generate/src/lib.rs
@@ -1,5 +1,6 @@
 extern crate proc_macro;
 
+mod errors;
 mod funcs;
 mod names;
 mod parse;
@@ -9,6 +10,7 @@ use proc_macro::TokenStream;
 use proc_macro2::TokenStream as TokenStream2;
 use quote::quote;
 
+use errors::define_error_trait;
 use funcs::define_func;
 use names::Names;
 use types::define_datatype;
@@ -22,29 +24,28 @@ pub fn from_witx(args: TokenStream) -> TokenStream {
 
     let doc = witx::load(&witx_paths).expect("loading witx");
 
-    let mut types = TokenStream2::new();
-    for namedtype in doc.typenames() {
-        let def = define_datatype(&names, &namedtype);
-        types.extend(def);
-    }
+    let types = doc.typenames().map(|t| define_datatype(&names, &t));
 
-    let mut modules = TokenStream2::new();
-    for module in doc.modules() {
+    let modules = doc.modules().map(|module| {
         let modname = names.module(&module.name);
-
-        let mut fs = TokenStream2::new();
-        for func in module.funcs() {
-            fs.extend(define_func(&names, &func));
-        }
-        modules.extend(quote!(
+        let fs = module.funcs().map(|f| define_func(&names, &f));
+        quote!(
             mod #modname {
                 use super::*;
                 use super::types::*;
                 use memory::*;
-                #fs
+                #(#fs)*
             }
-        ));
-    }
+        )
+    });
 
-    TokenStream::from(quote!(mod types { #types } #modules))
+    let error_trait = define_error_trait(&names, &doc);
+
+    TokenStream::from(quote!(
+        mod types {
+            #(#types)*
+            #error_trait
+        }
+        #(#modules)*
+    ))
 }
diff --git a/crates/generate/src/names.rs b/crates/generate/src/names.rs
index a674f13cfe..bd296ac88e 100644
--- a/crates/generate/src/names.rs
+++ b/crates/generate/src/names.rs
@@ -12,8 +12,9 @@ impl Names {
     pub fn new() -> Names {
         Names {}
     }
-    pub fn type_(&self, id: &Id) -> Ident {
-        format_ident!("{}", id.as_str().to_camel_case())
+    pub fn type_(&self, id: &Id) -> TokenStream {
+        let ident = format_ident!("{}", id.as_str().to_camel_case());
+        quote!(#ident)
     }
     pub fn builtin_type(&self, b: BuiltinType) -> TokenStream {
         match b {
diff --git a/crates/generate/src/types.rs b/crates/generate/src/types.rs
index e517134c9f..144b11f2e1 100644
--- a/crates/generate/src/types.rs
+++ b/crates/generate/src/types.rs
@@ -31,71 +31,68 @@ fn define_alias(names: &Names, name: &witx::Id, to: &witx::NamedType) -> TokenSt
 fn define_enum(names: &Names, name: &witx::Id, e: &witx::EnumDatatype) -> TokenStream {
     let ident = names.type_(&name);
 
-    let mut output = TokenStream::new();
     let repr = int_repr_tokens(e.repr);
-    output.extend(quote!(#[repr(#repr)]));
-    output.extend(quote!(#[derive(Copy, Clone, Debug, std::hash::Hash, Eq, PartialEq)]));
 
-    let mut variants = TokenStream::new();
-    let mut to_repr_cases = TokenStream::new();
-    let mut tryfrom_repr_cases = TokenStream::new();
-    for (n, variant) in e.variants.iter().enumerate() {
+    let variant_names = e.variants.iter().map(|v| names.enum_variant(&v.name));
+    let tryfrom_repr_cases = e.variants.iter().enumerate().map(|(n, v)| {
+        let variant_name = names.enum_variant(&v.name);
         let n = n as u32;
-        let variant_name = names.enum_variant(&variant.name);
-        variants.extend(quote!(#variant_name,));
-        tryfrom_repr_cases.extend(quote!(#n => Ok(#ident::#variant_name),));
-        to_repr_cases.extend(quote!(#ident::#variant_name => #n,));
-    }
+        quote!(#n => Ok(#ident::#variant_name))
+    });
+    let to_repr_cases = e.variants.iter().enumerate().map(|(n, v)| {
+        let variant_name = names.enum_variant(&v.name);
+        let n = n as u32;
+        quote!(#ident::#variant_name => #n)
+    });
 
-    tryfrom_repr_cases
-        .extend(quote!(_ => Err(::memory::GuestValueError::InvalidEnum(stringify!(#ident)))));
+    quote! {
+        #[repr(#repr)]
+        #[derive(Copy, Clone, Debug, ::std::hash::Hash, Eq, PartialEq)]
+        pub enum #ident {
+            #(#variant_names),*
+        }
 
-    output.extend(quote!(pub enum #ident {
-        #variants
-    }
+        impl ::std::convert::TryFrom<#repr> for #ident {
+            type Error = ::memory::GuestValueError;
+            fn try_from(value: #repr) -> Result<#ident, ::memory::GuestValueError> {
+                match value {
+                    #(#tryfrom_repr_cases),*,
+                    _ => Err(::memory::GuestValueError::InvalidEnum(stringify!(#ident))),
+                }
+            }
+        }
 
-    impl ::std::convert::TryFrom<#repr> for #ident {
-        type Error = ::memory::GuestValueError;
-        fn try_from(value: #repr) -> Result<#ident, ::memory::GuestValueError> {
-            match value {
-                #tryfrom_repr_cases
+        impl From<#ident> for #repr {
+            fn from(e: #ident) -> #repr {
+                match e {
+                    #(#to_repr_cases),*
+                }
+            }
+        }
+
+        impl ::memory::GuestType for #ident {
+            fn size() -> u32 {
+                ::std::mem::size_of::<#repr>() as u32
+            }
+            fn name() -> &'static str {
+                stringify!(#ident)
+            }
+        }
+
+        impl ::memory::GuestTypeCopy for #ident {
+            fn read_val(src: ::memory::GuestPtr<#ident>) -> Result<#ident, ::memory::GuestValueError> {
+                use ::std::convert::TryInto;
+                let val = unsafe { ::std::ptr::read_unaligned(src.ptr() as *const #repr) };
+                val.try_into()
+            }
+            fn write_val(val: #ident, dest: ::memory::GuestPtrMut<#ident>) {
+                let val: #repr = val.into();
+                unsafe {
+                    ::std::ptr::write_unaligned(dest.ptr_mut() as *mut #repr, val)
+                };
             }
         }
     }
-
-    impl From<#ident> for #repr {
-        fn from(e: #ident) -> #repr {
-            match e {
-                #to_repr_cases
-            }
-        }
-    }
-
-    impl ::memory::GuestType for #ident {
-        fn size() -> u32 {
-            ::std::mem::size_of::<#repr>() as u32
-        }
-        fn name() -> &'static str {
-            stringify!(#ident)
-        }
-    }
-
-    impl ::memory::GuestTypeCopy for #ident {
-        fn read_val(src: ::memory::GuestPtr<#ident>) -> Result<#ident, ::memory::GuestValueError> {
-            use ::std::convert::TryInto;
-            let val = unsafe { ::std::ptr::read_unaligned(src.ptr() as *const #repr) };
-            val.try_into()
-        }
-        fn write_val(val: #ident, dest: ::memory::GuestPtrMut<#ident>) {
-            let val: #repr = val.into();
-            unsafe {
-                ::std::ptr::write_unaligned(dest.ptr_mut() as *mut #repr, val)
-            };
-        }
-    }
-    ));
-
-    output
 }
 
 fn define_builtin(names: &Names, name: &witx::Id, builtin: witx::BuiltinType) -> TokenStream {
diff --git a/crates/memory/src/lib.rs b/crates/memory/src/lib.rs
index 6b8edeb287..efc2317a47 100644
--- a/crates/memory/src/lib.rs
+++ b/crates/memory/src/lib.rs
@@ -4,5 +4,5 @@ mod memory;
 mod region;
 
 pub use guest_type::{GuestType, GuestTypeClone, GuestTypeCopy, GuestValueError};
-pub use memory::{GuestMemory, GuestPtr, GuestPtrMut};
+pub use memory::{GuestMemory, GuestPtr, GuestPtrMut, MemoryError};
 pub use region::Region;
diff --git a/src/lib.rs b/src/lib.rs
index 227fdd8126..a146077675 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,6 +1,21 @@
 pub mod test {
-    pub struct WasiCtx {} // FIXME: parameterize macro on what ctx type is used here
     generate::from_witx!("test.witx");
+
+    pub struct WasiCtx {} // FIXME: parameterize macro on what ctx type is used here
+
+    impl types::WitxErrorConversion for WasiCtx {
+        fn success_to_errno(&mut self) -> types::Errno {
+            types::Errno::Ok
+        }
+        fn memory_error_to_errno(&mut self, e: ::memory::MemoryError) -> types::Errno {
+            eprintln!("memory error: {:?}", e);
+            types::Errno::InvalidArg
+        }
+        fn value_error_to_errno(&mut self, e: ::memory::GuestValueError) -> types::Errno {
+            eprintln!("guest value error: {:?}", e);
+            types::Errno::InvalidArg
+        }
+    }
 }
 /*
 pub mod wasi {
diff --git a/test.witx b/test.witx
index 1750ec3d15..940f8594d8 100644
--- a/test.witx
+++ b/test.witx
@@ -2,6 +2,8 @@
 
 (typename $errno
     (enum u32
+        $ok
+        $invalid_arg
         $dont_want_to
         $physically_unable
         $picket_line))