commit ef8f15a2e07cb784d5db796c147361bd1968ece0
Author: TudbuT <tudbut@tudbut.de>
Date:   Tue Nov 14 20:34:07 2023 +0100

    Initial commit

diff --git a/.gitignore b/.gitignore
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+/target
+/Cargo.lock
diff --git a/Cargo.toml b/Cargo.toml
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+[package]
+name = "type-fn"
+version = "0.1.0"
+edition = "2021"
+description = "Allows for simpler coding of type-level logic, e.g. for type-number systems."
+license = "MIT"
+repository = "https://github.com/tudbut/type-fn"
+
+[dependencies]
diff --git a/README.md b/README.md
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+# type-fn
+
+type-fn allows you to more simply create logic at the type level.
+
+## Example
+
+Unsigned addition, subtraction, and multiplication:
+
+```rs
+struct Zero;
+struct Succ<T>(PhantomData<T>);
+type_fn! {
+    fn Add<Lhs, Rhs>;
+    fn Sub<Lhs, Rhs>;
+    fn Mul<Lhs, Rhs>;
+}
+type_fn_impl! {
+    fn<TypeFn> Add< => Zero, Rhs> => Rhs;
+    fn<TypeFn> Add<N => Succ<N>, Rhs>
+        where
+            Add<N, Rhs>: + TypeFn,
+        => Succ<<Add<N, Rhs> as TypeFn>::Ret>;
+
+    fn<TypeFn> Sub<Lhs, => Zero> => Lhs;
+    fn<TypeFn> Sub<Lhs => Succ<Lhs>, Rhs => Succ<Rhs>>
+        where
+            Sub<Lhs, Rhs> : + TypeFn,
+        => <Sub<Lhs, Rhs> as TypeFn>::Ret;
+
+    fn<TypeFn> Mul< => Zero, Rhs> => Zero;
+    fn<TypeFn> Mul<Lhs => Succ<Lhs>, Rhs>
+        where
+            Mul<Lhs, Rhs>: + TypeFn,
+            Add<Rhs, <Mul<Lhs, Rhs> as TypeFn>::Ret>: + TypeFn,
+        => <Add<Rhs, <Mul<Lhs, Rhs> as TypeFn>::Ret> as TypeFn>::Ret;
+}
+```
diff --git a/src/lib.rs b/src/lib.rs
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+++ b/src/lib.rs
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+#![no_std]
+
+/// Default trait for returning something from a type-fn
+pub trait TypeFn {
+    type Ret;
+}
+
+/// Verifies equality between two types at compile-time.
+#[macro_export]
+macro_rules! assert_types_eq {
+    ($a:ty, $b:ty) => {{
+        let _: ::core::marker::PhantomData<$a> = ::core::marker::PhantomData::<$b>;
+    }};
+}
+
+/// Generates type-fn implementations.
+/// Syntax:
+/// `fn<$FnType$> $name$<$args$> $[$where-clause$]$ => $return-type$;`
+///
+/// FnType should simply be a trait containing `type Ret`, e.g. [`TypeFn`].
+///
+/// args is a type-arg list. To pattern-match types, use `T => Successor<T>` to
+/// implement something like like `Add<Successor<T>, Rhs>`.
+/// When implementing something for a specific type, use ` => TypeHere` (leave out
+/// the type arg name).
+/// If you need an extra type arg that wont be in the resulting type anywhere,
+/// add a bar `|` and write type args there as normal.
+///
+/// The where clause is just like rust's, except you need to put a plus before the first
+/// trait bound as well. Trailing commas are also mandatory.
+/// Trait bounds can only be set using the where-clause.
+///
+/// The return type can use any of the type arguments.
+///
+/// You can define an arbitrary amount of functions in one macro invocation.
+#[macro_export]
+macro_rules! type_fn_impl {
+    (@a_or_else_b  => ) => {compiler_error!()};
+    (@a_or_else_b  => $($b:ident)+) => {$($b)*};
+    (@a_or_else_b $($a:ty)+ => $($b:ident)*) => { $($a)+ };
+    ($(fn < $sup:ty > $name:ident <$($($arg:ident)? $(=> $($argv:ty)+)?),* $(| $($targ:ident),+)?>
+        $(where $($tv:ty : $( + $( ?$tcqm:ident )? $( $tc:ident )? )+ ,)+)? => $ret:ty;)+)
+    => {
+        $(
+            impl<$($($arg, )?)* $($($targ, )*)?>
+                $sup for $name <$($crate::type_fn_impl!(@a_or_else_b $($($argv)*)? => $($arg)?)),*>
+            $(where $($tv : $($(?$tcqm)? $($tc)? + )+ ),+)?
+            {
+                type Ret = $ret;
+            }
+        )+
+    };
+}
+
+/// Creates type functions. You will still need to implement them yourself, e.g. using
+/// [`type_fn_impl!`].
+/// Syntax:
+/// `$[$visibility$]$ fn $name$ <$args$>;`
+///
+/// visibility is just like rust's normal visibility modifier.
+///
+/// args is a list of type arguments. They can not have constraints at this time.
+#[macro_export]
+macro_rules! type_fn {
+    ($($vis:vis fn $name:ident <$($arg:ident),*>;)*) => {
+        $(
+            $vis struct $name <$($arg),*> ($(::core::marker::PhantomData<$arg>, )*);
+        )*
+    };
+}
+
+#[cfg(test)]
+mod tests {
+    use core::marker::PhantomData;
+
+    use crate::TypeFn;
+
+    #[test]
+    fn test_compile() {
+        struct Zero;
+        struct Succ<T>(PhantomData<T>);
+        type_fn! {
+            fn Add<Lhs, Rhs>;
+            fn Sub<Lhs, Rhs>;
+            fn Mul<Lhs, Rhs>;
+        }
+        type_fn_impl! {
+            fn<TypeFn> Add< => Zero, Rhs> => Rhs;
+            fn<TypeFn> Add<N => Succ<N>, Rhs>
+                where
+                    Add<N, Rhs>: + TypeFn,
+                => Succ<<Add<N, Rhs> as TypeFn>::Ret>;
+
+            fn<TypeFn> Sub<Lhs, => Zero> => Lhs;
+            fn<TypeFn> Sub<Lhs => Succ<Lhs>, Rhs => Succ<Rhs>>
+                where
+                    Sub<Lhs, Rhs> : + TypeFn,
+                => <Sub<Lhs, Rhs> as TypeFn>::Ret;
+
+            fn<TypeFn> Mul< => Zero, Rhs> => Zero;
+            fn<TypeFn> Mul<Lhs => Succ<Lhs>, Rhs>
+                where
+                    Mul<Lhs, Rhs>: + TypeFn,
+                    Add<Rhs, <Mul<Lhs, Rhs> as TypeFn>::Ret>: + TypeFn,
+                => <Add<Rhs, <Mul<Lhs, Rhs> as TypeFn>::Ret> as TypeFn>::Ret;
+        }
+
+        type TwoMinusOne = <Sub<Succ<Succ<Zero>>, Succ<Zero>> as TypeFn>::Ret;
+        assert_types_eq!(TwoMinusOne, Succ<Zero>);
+        assert_types_eq!(<Sub<TwoMinusOne, Succ<Zero>> as TypeFn>::Ret, Zero);
+        assert_types_eq!(
+            Succ<Succ<Succ<Succ<Zero>>>>,
+            <Mul<Succ<Succ<Zero>>, Succ<Succ<Zero>>> as TypeFn>::Ret
+        );
+        assert_types_eq!(Zero, <Mul<Succ<Succ<Zero>>, Zero> as TypeFn>::Ret);
+    }
+}