Add effect-as-a-clause syntax proposal

evaluation/syntax/effect-as-a-clause.md

@@ -0,0 +1,157 @@
+- Name: `effect-as-a-clause`
+- Proposed by: [@mominul](https://github.com/mominul) [@satvikpendem](https://github.com/satvikpendem)
+- Original proposal (optional): (https://github.com/rust-lang/keyword-generics-initiative/issues/14)
+
+# Design
+We want to propose the usage of the `effect` clause to achieve operation genericity, for example:
+```rust
+trait Read {
+    fn read(&mut self, buf: &mut [u8]) -> Result<usize>
+    effect
+        async;
+
+    fn read_to_string(&mut self, buf: &mut String) -> Result<usize> 
+    effect
+        async
+    { .. }
+}
+
+/// Function to read from the file into a string which may exhibit async or const effect
+fn read_to_string(path: &str) -> io::Result<String>
+effect
+       async, const 
+{
+    let mut string = String::new();
+
+    // We can be conditional over the context the function has been called from, 
+    // only when the function declaration has the `effect` clause
+    if async || !async {
+        let mut file = File::open("foo.txt")?; // File implements Read
+        // Because `read_to_string` is also an `effect` function that may or may not exhibit 
+        // async-ness par the declaration, we can use it on both contexts (async/sync) 
+        // we are placing the condition on.
+        file.read_to_string(&mut string)?;  // .await will be inferred.   
+    } else { // must be const
+        // As the `read_to_string` doesn't exhibit const-ness, we'll need to handle it ourselves.
+        string = include_str!(path).to_string();
+    }
+
+    Ok(string)
+}
+
+/// A normal function
+fn read() {
+    let data = read_to_string("hello.txt").unwrap();
+}
+
+/// A async function
+async fn read() {
+    let data = read_to_string("hello.txt").await.unwrap();
+}
+
+/// A const function
+const fn read() {
+    let data = read_to_string("hello.txt").unwrap();
+}
+```
+So in a nutshell, a function declaration with an `effect` clause is a special type of function that **may** or **may not** exhibit async or const behavior(effect) and it **depends on the context of the function being called from** and we can **execute a different piece of code according to the context** from the function was called from too (like the `const_eval_select`, resolves [#6](https://github.com/rust-lang/keyword-generics-initiative/issues/6)):
+
+```rust
+fn function() -> Result<()>
+effect
+    async, const
+{
+    // ...
+    if async {
+        // code for handling stuff asynchronously
+    } else if const {
+        // code for handling stuff `const`-way
+    else {
+        // code for handling stuff synchronously
+    }
+    // ...
+}
+```
+## base (reference)
+
+<!-- This is the snippet which is being translated to various scenarios we're
+translating from. Please keep this as-is, so we can reference it later.-->
+
+```rust
+/// A trimmed-down version of the `std::Iterator` trait.
+pub trait Iterator {
+    type Item;
+    fn next(&mut self) -> Option<Self::Item>;
+    fn size_hint(&self) -> (usize, Option<usize>);
+}
+
+/// An adaptation of `Iterator::find` to a free-function
+pub fn find<I, T, P>(iter: &mut I, predicate: P) -> Option<T>
+where
+    I: Iterator<Item = T> + Sized,
+    P: FnMut(&T) -> bool;
+```
+
+## always async
+
+<!-- A variant where all items are always `async` -->
+
+```rust
+pub trait Iterator {
+    type Item;
+    async fn next(&mut self) -> Option<Self::Item>;
+    fn size_hint(&self) -> (usize, Option<usize>);
+}
+
+pub async fn find<I, T, P>(iter: &mut I, predicate: P) -> Option<T>
+where
+    I: Iterator<Item = T> + Sized,
+    P: async FnMut(&T) -> bool;
+```
+
+## maybe async
+
+<!-- A variant where all items are generic over `async` -->
+
+```rust
+pub trait Iterator {
+    type Item;
+    fn next(&mut self) -> Option<Self::Item>
+    effect async;
+    fn size_hint(&self) -> (usize, Option<usize>);
+}
+
+pub fn find<I, T, P>(iter: &mut I, predicate: P) -> Option<T>
+where
+    I: Iterator<Item = T> + Sized,
+    P: FnMut(&T) -> bool effect async;
+effect
+    async
+```
+
+## generic over all modifier keywords
+
+<!-- A variant where all items are generic over all modifier keywords (e.g.
+`async`, `const`, `gen`, etc.) -->
+
+```rust
+pub trait Iterator {
+    type Item;
+    fn next(&mut self) -> Option<Self::Item>
+    effect async, const;
+    fn size_hint(&self) -> (usize, Option<usize>);
+}
+
+pub fn find<I, T, P>(iter: &mut I, predicate: P) -> Option<T>
+where
+    I: Iterator<Item = T> + Sized,
+    P: FnMut(&T) -> bool effect async, const;
+effect
+    async, const
+```
+
+# Notes
+
+<!-- Add additional notes, context, and thoughts you want to share about your design
+here -->
+We can introduce `maybe` keyword instead of `effect` if it seems more appropriate terminology for the semantics described in this proposal.