Add macro bindings for views

crates/bindings-macro/src/lib.rs

@@ -12,6 +12,7 @@ mod reducer;
 mod sats;
 mod table;
 mod util;
+mod view;
 
 use proc_macro::TokenStream as StdTokenStream;
 use proc_macro2::TokenStream;
@@ -112,6 +113,14 @@ pub fn reducer(args: StdTokenStream, item: StdTokenStream) -> StdTokenStream {
     })
 }
 
+#[proc_macro_attribute]
+pub fn view(args: StdTokenStream, item: StdTokenStream) -> StdTokenStream {
+    cvt_attr::<ItemFn>(args, item, quote!(), |args, original_function| {
+        let args = view::ViewArgs::parse(args)?;
+        view::view_impl(args, original_function)
+    })
+}
+
 /// It turns out to be shockingly difficult to construct an [`Attribute`].
 /// That type is not [`Parse`], instead having two distinct methods
 /// for parsing "inner" vs "outer" attributes.

crates/bindings-macro/src/reducer.rs

@@ -139,11 +139,12 @@ pub(crate) fn reducer_impl(args: ReducerArgs, original_function: &ItemFn) -> syn
             }
         }
         #[automatically_derived]
-        impl spacetimedb::rt::ReducerInfo for #func_name {
+        impl spacetimedb::rt::FnInfo for #func_name {
+            type Invoke = spacetimedb::rt::ReducerFn;
             const NAME: &'static str = #reducer_name;
             #(const LIFECYCLE: Option<spacetimedb::rt::LifecycleReducer> = Some(#lifecycle);)*
             const ARG_NAMES: &'static [Option<&'static str>] = &[#(#opt_arg_names),*];
-            const INVOKE: spacetimedb::rt::ReducerFn = #func_name::invoke;
+            const INVOKE: Self::Invoke = #func_name::invoke;
         }
     })
 }

crates/bindings-macro/src/table.rs

@@ -821,7 +821,7 @@ pub(crate) fn table_impl(mut args: TableArgs, item: &syn::DeriveInput) -> syn::R
             let reducer = &sched.reducer;
             let scheduled_at_id = scheduled_at_column.index;
             let desc = quote!(spacetimedb::table::ScheduleDesc {
-                reducer_name: <#reducer as spacetimedb::rt::ReducerInfo>::NAME,
+                reducer_name: <#reducer as spacetimedb::rt::FnInfo>::NAME,
                 scheduled_at_column: #scheduled_at_id,
             });
 

crates/bindings-macro/src/view.rs

@@ -0,0 +1,178 @@
+use proc_macro2::{Span, TokenStream};
+use quote::quote;
+use syn::parse::Parser;
+use syn::{FnArg, ItemFn};
+
+use crate::sym;
+use crate::util::{ident_to_litstr, match_meta};
+
+pub(crate) struct ViewArgs {
+    #[allow(unused)]
+    public: bool,
+}
+
+impl ViewArgs {
+    /// Parse `#[view(public)]` where `public` is required.
+    pub(crate) fn parse(input: TokenStream) -> syn::Result<Self> {
+        if input.is_empty() {
+            return Err(syn::Error::new(
+                Span::call_site(),
+                "views must be declared as `#[view(public)]`; `public` is required",
+            ));
+        }
+        let mut public = false;
+        syn::meta::parser(|meta| {
+            match_meta!(match meta {
+                sym::public => {
+                    if public {
+                        return Err(syn::Error::new(
+                            Span::call_site(),
+                            "duplicate attribute argument: `public`",
+                        ));
+                    }
+                    public = true;
+                }
+            });
+            Ok(())
+        })
+        .parse2(input)?;
+        if !public {
+            return Err(syn::Error::new(
+                Span::call_site(),
+                "views must be declared as `#[view(public)]`; `public` is required",
+            ));
+        }
+        Ok(Self { public })
+    }
+}
+
+pub(crate) fn view_impl(_args: ViewArgs, original_function: &ItemFn) -> syn::Result<TokenStream> {
+    let func_name = &original_function.sig.ident;
+    let view_name = ident_to_litstr(func_name);
+    let vis = &original_function.vis;
+
+    for param in &original_function.sig.generics.params {
+        let err = |msg| syn::Error::new_spanned(param, msg);
+        match param {
+            syn::GenericParam::Lifetime(_) => {}
+            syn::GenericParam::Type(_) => return Err(err("type parameters are not allowed on views")),
+            syn::GenericParam::Const(_) => return Err(err("const parameters are not allowed on views")),
+        }
+    }
+
+    // Extract parameters
+    let typed_args = original_function
+        .sig
+        .inputs
+        .iter()
+        .map(|arg| match arg {
+            FnArg::Typed(arg) => Ok(arg),
+            FnArg::Receiver(_) => Err(syn::Error::new_spanned(
+                arg,
+                "The `self` parameter is not allowed in views",
+            )),
+        })
+        .collect::<syn::Result<Vec<_>>>()?;
+
+    // Extract parameter names
+    let opt_arg_names = typed_args.iter().map(|arg| {
+        if let syn::Pat::Ident(i) = &*arg.pat {
+            let name = i.ident.to_string();
+            quote!(Some(#name))
+        } else {
+            quote!(None)
+        }
+    });
+
+    let arg_tys = typed_args.iter().map(|arg| arg.ty.as_ref()).collect::<Vec<_>>();
+
+    // Extract the context type and the rest of the parameter types
+    let [ctx_ty, arg_tys @ ..] = &arg_tys[..] else {
+        return Err(syn::Error::new_spanned(
+            &original_function.sig,
+            "Views must always have a context parameter: `&ViewContext` or `&AnonymousViewContext`",
+        ));
+    };
+
+    // Extract the context type
+    let ctx_ty = match ctx_ty {
+        syn::Type::Reference(ctx_ty) => ctx_ty.elem.as_ref(),
+        _ => {
+            return Err(syn::Error::new_spanned(
+                ctx_ty,
+                "The first parameter of a view must be a context parameter: `&ViewContext` or `&AnonymousViewContext`; passed by reference",
+            ));
+        }
+    };
+
+    // Views must return a result
+    let ret_ty = match &original_function.sig.output {
+        syn::ReturnType::Type(_, t) => t.as_ref(),
+        syn::ReturnType::Default => {
+            return Err(syn::Error::new_spanned(
+                &original_function.sig,
+                "views must return `Vec<T>` or `Option<T>` where `T` is a `SpacetimeType`",
+            ));
+        }
+    };
+
+    let register_describer_symbol = format!("__preinit__20_register_describer_{}", view_name.value());
+
+    let lt_params = &original_function.sig.generics;
+    let lt_where_clause = &lt_params.where_clause;
+
+    let generated_describe_function = quote! {
+        #[export_name = #register_describer_symbol]
+        pub extern "C" fn __register_describer() {
+            spacetimedb::rt::ViewRegistrar::<#ctx_ty>::register::<_, #func_name, _, _>(#func_name)
+        }
+    };
+
+    Ok(quote! {
+        const _: () = { #generated_describe_function };
+
+        #[allow(non_camel_case_types)]
+        #vis struct #func_name { _never: ::core::convert::Infallible }
+
+        const _: () = {
+            fn _assert_args #lt_params () #lt_where_clause {
+                let _ = <#ctx_ty as spacetimedb::rt::ViewContextArg>::_ITEM;
+                let _ = <#ret_ty as spacetimedb::rt::ViewReturn>::_ITEM;
+            }
+        };
+
+        const _: () = {
+            fn _assert_args #lt_params () #lt_where_clause {
+                #(let _ = <#arg_tys as spacetimedb::rt::ViewArg>::_ITEM;)*
+            }
+        };
+
+        impl #func_name {
+            fn invoke(__ctx: #ctx_ty, __args: &[u8]) -> Vec<u8> {
+                spacetimedb::rt::ViewDispatcher::<#ctx_ty>::invoke::<_, _, _>(#func_name, __ctx, __args)
+            }
+        }
+
+        #[automatically_derived]
+        impl spacetimedb::rt::FnInfo for #func_name {
+            /// The type of this function
+            type Invoke = <spacetimedb::rt::ViewKind<#ctx_ty> as spacetimedb::rt::ViewKindTrait>::InvokeFn;
+
+            /// The name of this function
+            const NAME: &'static str = #view_name;
+
+            /// The parameter names of this function
+            const ARG_NAMES: &'static [Option<&'static str>] = &[#(#opt_arg_names),*];
+
+            /// The pointer for invoking this function
+            const INVOKE: Self::Invoke = #func_name::invoke;
+
+            /// The return type of this function
+            fn return_type(
+                ts: &mut impl spacetimedb::sats::typespace::TypespaceBuilder
+            ) -> Option<spacetimedb::sats::AlgebraicType> {
+                Some(<#ret_ty as spacetimedb::SpacetimeType>::make_type(ts))
+            }
+        }
+    })
+}

crates/bindings/src/lib.rs

@@ -667,6 +667,133 @@ pub use spacetimedb_bindings_macro::table;
 #[doc(inline)]
 pub use spacetimedb_bindings_macro::reducer;
 
+/// Marks a function as a spacetimedb view.
+///
+/// A view is a function with read-only access to the database.
+///
+/// The first argument of a view is always a [`&ViewContext`] or [`&AnonymousViewContext`].
+/// The former can only read from the database whereas latter can also access info about the caller.
+///
+/// After this, a view can take any number of arguments just like reducers.
+/// These arguments must implement the [`SpacetimeType`], [`Serialize`], and [`Deserialize`] traits.
+/// All of these traits can be derived at once by marking a type with `#[derive(SpacetimeType)]`.
+///
+/// Views return `Vec<T>` or `Option<T>` where `T` is a `SpacetimeType`.
+///
+/// ```no_run
+/// # mod demo {
+/// use spacetimedb::{view, table, AnonymousViewContext, SpacetimeType, ViewContext};
+/// use spacetimedb_lib::Identity;
+///
+/// #[table(name = player)]
+/// struct Player {
+///     #[auto_inc]
+///     #[primary_key]
+///     id: u64,
+///
+///     #[unique]
+///     identity: Identity,
+///
+///     #[index(btree)]
+///     level: u32,
+/// }
+///
+/// impl Player {
+///     fn merge(self, location: Location) -> PlayerAndLocation {
+///         PlayerAndLocation {
+///             player_id: self.id,
+///             level: self.level,
+///             x: location.x,
+///             y: location.y,
+///         }
+///     }
+/// }
+///
+/// #[derive(SpacetimeType)]
+/// struct PlayerId {
+///     id: u64,
+/// }
+///
+/// #[table(name = location, index(name = coordinates, btree(columns = [x, y])))]
+/// struct Location {
+///     #[unique]
+///     player_id: u64,
+///     x: u64,
+///     y: u64,
+/// }
+///
+/// #[derive(SpacetimeType)]
+/// struct PlayerAndLocation {
+///     player_id: u64,
+///     level: u32,
+///     x: u64,
+///     y: u64,
+/// }
+///
+/// // A view that selects at most one row from a table
+/// #[view(public)]
+/// fn my_player(ctx: &ViewContext) -> Option<Player> {
+///     ctx.db.player().identity().find(ctx.sender)
+/// }
+///
+/// // An example of column projection
+/// #[view(public)]
+/// fn my_player_id(ctx: &ViewContext) -> Option<PlayerId> {
+///     ctx.db.player().identity().find(ctx.sender).map(|Player { id, .. }| PlayerId { id })
+/// }
+///
+/// // An example of a parameterized view
+/// #[view(public)]
+/// fn players_at_level(ctx: &AnonymousViewContext, level: u32) -> Vec<Player> {
+///     ctx.db.player().level().filter(level).collect()
+/// }
+///
+/// // An example that is analogous to a semijoin in sql
+/// #[view(public)]
+/// fn players_at_coordinates(ctx: &AnonymousViewContext, x: u64, y: u64) -> Vec<Player> {
+///     ctx
+///         .db
+///         .location()
+///         .coordinates()
+///         .filter((x, y))
+///         .filter_map(|location| ctx.db.player().id().find(location.player_id))
+///         .collect()
+/// }
+///
+/// // An example of a join that combines fields from two different tables
+/// #[view(public)]
+/// fn players_with_coordinates(ctx: &AnonymousViewContext, x: u64, y: u64) -> Vec<PlayerAndLocation> {
+///     ctx
+///         .db
+///         .location()
+///         .coordinates()
+///         .filter((x, y))
+///         .filter_map(|location| ctx
+///             .db
+///             .player()
+///             .id()
+///             .find(location.player_id)
+///             .map(|player| player.merge(location))
+///         )
+///         .collect()
+/// }
+/// # }
+/// ```
+///
+/// Just like reducers, views are limited in their ability to interact with the outside world.
+/// They have no access to any network or filesystem interfaces.
+/// Calling methods from [`std::io`], [`std::net`], or [`std::fs`] will result in runtime errors.
+///
+/// Views are callable by reducers and other views simply by passing their `ViewContext`..
+/// This is a regular function call.
+/// The callee will run within the caller's transaction.
+///
+///
+/// [`&ViewContext`]: `ViewContext`
+/// [`&AnonymousViewContext`]: `AnonymousViewContext`
+#[doc(inline)]
+pub use spacetimedb_bindings_macro::view;
+
 /// One of two possible types that can be passed as the first argument to a `#[view]`.
 /// The other is [`ViewContext`].
 /// Use this type if the view does not depend on the caller's identity.