diff --git a/crates/generate/src/config.rs b/crates/generate/src/config.rs
index 3cb1cf5c09..9e7848e82c 100644
--- a/crates/generate/src/config.rs
+++ b/crates/generate/src/config.rs
@@ -1,64 +1,98 @@
 use std::path::PathBuf;
 
+use proc_macro2::Span;
 use syn::{
-    bracketed,
+    braced, bracketed,
     parse::{Parse, ParseStream},
     punctuated::Punctuated,
-    token, LitStr, Result, Token,
+    Error, Ident, LitStr, Result, Token,
 };
 
+#[derive(Debug, Clone)]
 pub struct Config {
-    _bracket_token: token::Bracket,
-    path_lits: Punctuated<LitStr, Token![,]>,
+    pub witx: WitxConf,
+    pub ctx: CtxConf,
+}
+
+enum ConfigField {
+    Witx(WitxConf),
+    Ctx(CtxConf),
+}
+
+impl Parse for ConfigField {
+    fn parse(input: ParseStream) -> Result<Self> {
+        let id: Ident = input.parse()?;
+        let _colon: Token![:] = input.parse()?;
+        match id.to_string().as_ref() {
+            "witx" => Ok(ConfigField::Witx(input.parse()?)),
+            "ctx" => Ok(ConfigField::Ctx(input.parse()?)),
+            _ => Err(Error::new(id.span(), "expected `witx` or `ctx`")),
+        }
+    }
 }
 
 impl Config {
-    pub fn witx_paths(&self) -> Vec<PathBuf> {
-        self.path_lits
-            .iter()
-            .map(|lit| PathBuf::from(lit.value()))
-            .collect()
+    fn build(fields: impl Iterator<Item = ConfigField>, err_loc: Span) -> Result<Self> {
+        let mut witx = None;
+        let mut ctx = None;
+        for f in fields {
+            match f {
+                ConfigField::Witx(c) => {
+                    witx = Some(c);
+                }
+                ConfigField::Ctx(c) => {
+                    ctx = Some(c);
+                }
+            }
+        }
+        Ok(Config {
+            witx: witx
+                .take()
+                .ok_or_else(|| Error::new(err_loc, "`witx` field required"))?,
+            ctx: ctx
+                .take()
+                .ok_or_else(|| Error::new(err_loc, "`ctx` field required"))?,
+        })
     }
 }
 
 impl Parse for Config {
+    fn parse(input: ParseStream) -> Result<Self> {
+        let contents;
+        let _lbrace = braced!(contents in input);
+        let fields: Punctuated<ConfigField, Token![,]> =
+            contents.parse_terminated(ConfigField::parse)?;
+        Ok(Config::build(fields.into_iter(), input.span())?)
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct WitxConf {
+    pub paths: Vec<PathBuf>,
+}
+
+impl Parse for WitxConf {
     fn parse(input: ParseStream) -> Result<Self> {
         let content;
-        Ok(Config {
-            _bracket_token: bracketed!(content in input),
-            path_lits: content.parse_terminated(Parse::parse)?,
+        let _ = bracketed!(content in input);
+        let path_lits: Punctuated<LitStr, Token![,]> = content.parse_terminated(Parse::parse)?;
+        let paths: Vec<PathBuf> = path_lits
+            .iter()
+            .map(|lit| PathBuf::from(lit.value()))
+            .collect();
+        Ok(WitxConf { paths })
+    }
+}
+
+#[derive(Debug, Clone)]
+pub struct CtxConf {
+    pub name: Ident,
+}
+
+impl Parse for CtxConf {
+    fn parse(input: ParseStream) -> Result<Self> {
+        Ok(CtxConf {
+            name: input.parse()?,
         })
     }
 }
-
-/*
-    let arg_strings = args
-        .into_iter()
-        .map(|arg| match arg {
-            TokenTree::Literal(lit) => string_literal(lit),
-            _ => bail!("expected string literal, got: {:?}", arg),
-        })
-        .collect::<Result<Vec<String>>>()?;
-
-    if arg_strings.is_empty() {
-        bail!("expected at least one argument");
-    }
-    Ok(arg_strings)
-}
-
-fn string_literal(literal: Literal) -> Result<String> {
-    let s = literal.to_string();
-    if !s.starts_with('"') || !s.ends_with('"') {
-        bail!("string literal must be enclosed in double quotes");
-    }
-
-    let trimmed = s[1..s.len() - 1].to_owned();
-    if trimmed.contains('"') {
-        bail!("string literal must not contain quotes");
-    }
-    if trimmed.contains('\\') {
-        bail!("string literal must not contain backslashes");
-    }
-    Ok(trimmed)
-}
-*/
diff --git a/crates/generate/src/funcs.rs b/crates/generate/src/funcs.rs
index 15f3a8fae5..8313fe0c1c 100644
--- a/crates/generate/src/funcs.rs
+++ b/crates/generate/src/funcs.rs
@@ -4,11 +4,9 @@ use quote::quote;
 use crate::names::Names;
 use crate::types::struct_is_copy;
 
-// 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.
-//
 pub fn define_func(names: &Names, func: &witx::InterfaceFunc) -> TokenStream {
     let ident = names.func(&func.name);
+    let ctx_type = names.ctx_type();
     let coretype = func.core_type();
 
     let params = coretype.args.iter().map(|arg| match arg.signifies {
@@ -30,7 +28,7 @@ pub fn define_func(names: &Names, func: &witx::InterfaceFunc) -> TokenStream {
     });
 
     let abi_args = quote!(
-            ctx: &mut WasiCtx, memory: ::memory::GuestMemory,
+            ctx: &mut #ctx_type, memory: ::memory::GuestMemory,
             #(#params),*
     );
     let abi_ret = if let Some(ret) = &coretype.ret {
diff --git a/crates/generate/src/lib.rs b/crates/generate/src/lib.rs
index 9b66c9e1e8..4a50d88874 100644
--- a/crates/generate/src/lib.rs
+++ b/crates/generate/src/lib.rs
@@ -20,9 +20,9 @@ use types::define_datatype;
 pub fn from_witx(args: TokenStream) -> TokenStream {
     let config = parse_macro_input!(args as Config);
 
-    let names = Names::new(); // TODO parse the names from the invocation of the macro, or from a file?
+    let doc = witx::load(&config.witx.paths).expect("loading witx");
 
-    let doc = witx::load(&config.witx_paths()).expect("loading witx");
+    let names = Names::new(config); // TODO parse the names from the invocation of the macro, or from a file?
 
     let types = doc.typenames().map(|t| define_datatype(&names, &t));
 
@@ -30,9 +30,10 @@ pub fn from_witx(args: TokenStream) -> TokenStream {
         let modname = names.module(&module.name);
         let fs = module.funcs().map(|f| define_func(&names, &f));
         let modtrait = define_module_trait(&names, &module);
+        let ctx_type = names.ctx_type();
         quote!(
             mod #modname {
-                use super::WasiCtx;
+                use super::#ctx_type;
                 use super::types::*;
                 #(#fs)*
 
diff --git a/crates/generate/src/names.rs b/crates/generate/src/names.rs
index 558a7e28dc..c9ad2ba253 100644
--- a/crates/generate/src/names.rs
+++ b/crates/generate/src/names.rs
@@ -3,14 +3,19 @@ use proc_macro2::{Ident, TokenStream};
 use quote::{format_ident, quote};
 use witx::{AtomType, BuiltinType, Id, TypeRef};
 
+use crate::Config;
+
 #[derive(Debug, Clone)]
 pub struct Names {
-    // FIXME: overrides go in here, so we can map e.g. 2big => TooBig
+    config: Config,
 }
 
 impl Names {
-    pub fn new() -> Names {
-        Names {}
+    pub fn new(config: Config) -> Names {
+        Names { config }
+    }
+    pub fn ctx_type(&self) -> Ident {
+        self.config.ctx.name.clone()
     }
     pub fn type_(&self, id: &Id) -> TokenStream {
         let ident = format_ident!("{}", id.as_str().to_camel_case());
diff --git a/src/lib.rs b/src/lib.rs
index 65356c89a5..145e6ea783 100644
--- a/src/lib.rs
+++ b/src/lib.rs
@@ -1,6 +1,9 @@
 pub mod test {
     // FIXME: parameterize macro on what ctx type is used here
-    generate::from_witx!(["test.witx"]);
+    generate::from_witx!({
+        witx: ["test.witx"],
+        ctx: WasiCtx,
+    });
 
     pub struct WasiCtx {
         guest_errors: Vec<::memory::GuestError>,