diff --git a/crates/generate/src/funcs.rs b/crates/generate/src/funcs.rs
index bfe720f2d9..255e7a90e9 100644
--- a/crates/generate/src/funcs.rs
+++ b/crates/generate/src/funcs.rs
@@ -1,4 +1,4 @@
-use proc_macro2::TokenStream;
+use proc_macro2::{Ident, TokenStream};
 use quote::quote;
 
 use crate::names::Names;
@@ -6,10 +6,6 @@ use crate::names::Names;
 // 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.
 //
-// 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);
 
@@ -38,7 +34,7 @@ pub fn define_func(names: &Names, func: &witx::InterfaceFunc) -> TokenStream {
         .chain(func.results.iter().skip(1))
         .map(arg_signature);
     let abi_args = quote!(
-            wasi_ctx: &mut WasiCtx, memory: GuestMemory,
+            ctx: &mut WasiCtx, memory: ::memory::GuestMemory,
             #(#params),*
     );
     let abi_ret = if let Some(first_result) = func.results.get(0) {
@@ -47,10 +43,70 @@ pub fn define_func(names: &Names, func: &witx::InterfaceFunc) -> TokenStream {
             _ => unreachable!("first result should always be passed by value"),
         }
     } else if func.noreturn {
-        quote!(!)
+        // Ideally we would return `quote!(!)` here, but, we'd have to change
+        // the error handling logic in all the marshalling code to never return,
+        // and instead provide some other way to bail to the context...
+        // noreturn func
+        unimplemented!("noreturn funcs not supported yet!")
     } else {
         quote!(())
     };
 
-    quote!(pub fn #ident(#abi_args) -> #abi_ret { unimplemented!() })
+    let err_type = func
+        .results
+        .get(0)
+        .map(|res| names.type_ref(&res.tref))
+        .unwrap_or_else(|| abi_ret.clone());
+    let err_val = func
+        .results
+        .get(0)
+        .map(|_res| quote!(#abi_ret::from(e)))
+        .unwrap_or_else(|| quote!(()));
+
+    let marshal_args = func
+        .params
+        .iter()
+        .map(|param| match param.tref.type_().passed_by() {
+            witx::TypePassedBy::Value(_atom) => {
+                // FIXME atom -> param.tref can be either an `as` conversion, or `try_from`
+                let name = names.func_param(&param.name);
+                let interface_type = names.type_ref(&param.tref);
+                quote!( let #name = #name as #interface_type; )
+            }
+            _ => unimplemented!(),
+        });
+    let trait_args = func
+        .params
+        .iter()
+        .map(|param| names.func_param(&param.name));
+
+    let trait_rets = func
+        .results
+        .iter()
+        .skip(1)
+        .map(|result| names.func_param(&result.name))
+        .collect::<Vec<Ident>>();
+    let (trait_rets, trait_bindings) = if trait_rets.is_empty() {
+        (quote!({}), quote!(_))
+    } else {
+        let tuple = quote!((#(#trait_rets),*));
+        (tuple.clone(), tuple)
+    };
+
+    let marshal_rets = func
+        .results
+        .iter()
+        .skip(1)
+        .map(|_result| quote! { unimplemented!("convert result..."); });
+
+    quote!(pub fn #ident(#abi_args) -> #abi_ret {
+        #(#marshal_args)*
+        let #trait_bindings  = match ctx.#ident(#(#trait_args),*) {
+            Ok(#trait_bindings) => #trait_rets,
+            Err(e) => { return #err_val; },
+        };
+        #(#marshal_rets)*
+        let success:#err_type = ::memory::GuestError::success();
+        #abi_ret::from(success)
+    })
 }
diff --git a/crates/generate/src/lib.rs b/crates/generate/src/lib.rs
index 76e1014c7e..49d9289d58 100644
--- a/crates/generate/src/lib.rs
+++ b/crates/generate/src/lib.rs
@@ -1,6 +1,7 @@
 extern crate proc_macro;
 
 mod funcs;
+mod module_trait;
 mod names;
 mod parse;
 mod types;
@@ -10,6 +11,7 @@ use proc_macro2::TokenStream as TokenStream2;
 use quote::quote;
 
 use funcs::define_func;
+use module_trait::define_module_trait;
 use names::Names;
 use types::define_datatype;
 
@@ -27,12 +29,14 @@ pub fn from_witx(args: TokenStream) -> TokenStream {
     let modules = doc.modules().map(|module| {
         let modname = names.module(&module.name);
         let fs = module.funcs().map(|f| define_func(&names, &f));
+        let modtrait = define_module_trait(&names, &module);
         quote!(
             mod #modname {
-                use super::*;
+                use super::WasiCtx;
                 use super::types::*;
-                use memory::*;
                 #(#fs)*
+
+                #modtrait
             }
         )
     });
diff --git a/crates/generate/src/module_trait.rs b/crates/generate/src/module_trait.rs
new file mode 100644
index 0000000000..a25addd710
--- /dev/null
+++ b/crates/generate/src/module_trait.rs
@@ -0,0 +1,33 @@
+use proc_macro2::TokenStream;
+use quote::quote;
+
+use crate::names::Names;
+use witx::Module;
+
+pub fn define_module_trait(names: &Names, m: &Module) -> TokenStream {
+    let traitname = names.trait_name(&m.name);
+    let traitmethods = m.funcs().map(|f| {
+        let funcname = names.func(&f.name);
+        let args = f.params.iter().map(|arg| {
+            let arg_name = names.func_param(&arg.name);
+            let arg_type = names.type_ref(&arg.tref);
+            quote!(#arg_name: #arg_type)
+        });
+        let rets = f
+            .results
+            .iter()
+            .skip(1)
+            .map(|ret| names.type_ref(&ret.tref));
+        let err = f
+            .results
+            .get(0)
+            .map(|err_result| names.type_ref(&err_result.tref))
+            .unwrap_or(quote!(()));
+        quote!(fn #funcname(&mut self, #(#args),*) -> Result<(#(#rets),*), #err>;)
+    });
+    quote! {
+        pub trait #traitname {
+            #(#traitmethods)*
+        }
+    }
+}
diff --git a/crates/generate/src/names.rs b/crates/generate/src/names.rs
index bd296ac88e..78cec1e0b8 100644
--- a/crates/generate/src/names.rs
+++ b/crates/generate/src/names.rs
@@ -42,7 +42,6 @@ impl Names {
         }
     }
 
-    #[allow(unused)]
     pub fn type_ref(&self, tref: &TypeRef) -> TokenStream {
         match tref {
             TypeRef::Name(nt) => {
@@ -69,6 +68,10 @@ impl Names {
         format_ident!("{}", id.as_str().to_snake_case())
     }
 
+    pub fn trait_name(&self, id: &Id) -> Ident {
+        format_ident!("{}", id.as_str().to_camel_case())
+    }
+
     pub fn func(&self, id: &Id) -> Ident {
         format_ident!("{}", id.as_str().to_snake_case())
     }
diff --git a/crates/generate/src/types.rs b/crates/generate/src/types.rs
index 144b11f2e1..7d34f9688d 100644
--- a/crates/generate/src/types.rs
+++ b/crates/generate/src/types.rs
@@ -32,6 +32,7 @@ fn define_enum(names: &Names, name: &witx::Id, e: &witx::EnumDatatype) -> TokenS
     let ident = names.type_(&name);
 
     let repr = int_repr_tokens(e.repr);
+    let signed_repr = int_signed_repr_tokens(e.repr);
 
     let variant_names = e.variants.iter().map(|v| names.enum_variant(&v.name));
     let tryfrom_repr_cases = e.variants.iter().enumerate().map(|(n, v)| {
@@ -62,6 +63,13 @@ fn define_enum(names: &Names, name: &witx::Id, e: &witx::EnumDatatype) -> TokenS
             }
         }
 
+        impl ::std::convert::TryFrom<#signed_repr> for #ident {
+            type Error = ::memory::GuestValueError;
+            fn try_from(value: #signed_repr) -> Result<#ident, ::memory::GuestValueError> {
+                #ident::try_from(value as #repr)
+            }
+        }
+
         impl From<#ident> for #repr {
             fn from(e: #ident) -> #repr {
                 match e {
@@ -70,6 +78,12 @@ fn define_enum(names: &Names, name: &witx::Id, e: &witx::EnumDatatype) -> TokenS
             }
         }
 
+        impl From<#ident> for #signed_repr {
+            fn from(e: #ident) -> #signed_repr {
+                #repr::from(e) as #signed_repr
+            }
+        }
+
         impl ::memory::GuestType for #ident {
             fn size() -> u32 {
                 ::std::mem::size_of::<#repr>() as u32
@@ -109,3 +123,11 @@ fn int_repr_tokens(int_repr: witx::IntRepr) -> TokenStream {
         witx::IntRepr::U64 => quote!(u64),
     }
 }
+fn int_signed_repr_tokens(int_repr: witx::IntRepr) -> TokenStream {
+    match int_repr {
+        witx::IntRepr::U8 => quote!(i8),
+        witx::IntRepr::U16 => quote!(i16),
+        witx::IntRepr::U32 => quote!(i32),
+        witx::IntRepr::U64 => quote!(i64),
+    }
+}
diff --git a/crates/memory/src/guest_type.rs b/crates/memory/src/guest_type.rs
index 933b11c33e..9410e6054d 100644
--- a/crates/memory/src/guest_type.rs
+++ b/crates/memory/src/guest_type.rs
@@ -69,7 +69,7 @@ builtin_copy!(u8, i8, u16, i16, u32, i32, u64, i64, f32, f64, usize, char);
 
 pub trait GuestError {
     type Context;
-    fn is_success(&self) -> bool;
+    fn success() -> Self;
     fn from_memory_error(memory_error: MemoryError, ctx: &mut Self::Context) -> Self;
     fn from_value_error(value_error: GuestValueError, ctx: &mut Self::Context) -> Self;
 }
diff --git a/src/lib.rs b/src/lib.rs
index cdb98b3dd4..79e3ebc7c7 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -7,17 +7,21 @@ pub mod test {
         value_errors: Vec<::memory::GuestValueError>,
     }
 
+    impl foo::Foo for WasiCtx {
+        fn bar(&mut self, an_int: u32, an_float: f32) -> Result<(), types::Errno> {
+            println!("BAR: {} {}", an_int, an_float);
+            Ok(())
+        }
+    }
+
     // Errno is used as a first return value in the functions above, therefore
     // it must implement GuestError with type Context = WasiCtx.
     // The context type should let you do logging or debugging or whatever you need
     // with these errors. We just push them to vecs.
     impl ::memory::GuestError for types::Errno {
         type Context = WasiCtx;
-        fn is_success(&self) -> bool {
-            match self {
-                types::Errno::Ok => true,
-                _ => false,
-            }
+        fn success() -> types::Errno {
+            types::Errno::Ok
         }
         fn from_memory_error(e: ::memory::MemoryError, ctx: &mut WasiCtx) -> types::Errno {
             ctx.mem_errors.push(e);