feat: support simple equal format

Cargo.toml

@@ -2,7 +2,16 @@
 name = "merge-cfg"
 version = "0.1.0"
 edition = "2021"
+authors = ["sasakiyori <[email protected]>"]
+license = "MIT"
+description = "Merge or cover config based on priorities."
+keywords = ["config"]
 
 # See more keys and their definitions at https://doc.rust-lang.org/cargo/reference/manifest.html
 
+[lib]
+proc-macro = true
+
 [dependencies]
+syn = "2.0"
+quote = "1.0"

README.md

@@ -1,2 +1,59 @@
 # merge-cfg
+
 Merge or cover config based on priorities.
+
+## Usage
+
+You can get your configuration from config file in any way at first. Then derive `MergeCfg` to merge your configuration from command line arguments.
+
+```rust
+use merge_cfg::MergeCfg;
+
+#[derive(Debug, MergeCfg)]
+struct Config {
+    id: u64,
+    name: String,
+}
+
+fn main() {
+    let mut cfg = Config {
+        id: 1,
+        name: "abc".to_string(),
+    };
+    cfg.merge_cfg();
+    println!("{:?}", cfg);
+}
+```
+
+Command line arguments format: `{field}={value}`, field name should be the same with what you defined in your structure:
+
+```shell
+$ cargo run
+Config { id: 1, name: "abc" }
+
+$ cargo run id=2
+Config { id: 2, name: "abc" }
+
+$ cargo run name=xyz
+Config { id: 1, name: "xyz" }
+
+$ cargo run id=2 name=xyz
+Config { id: 2, name: "xyz" }
+```
+
+## Roadmap
+
+- [ ] Support environment variables
+  - [ ] Merge priority
+  - [ ] Merge option (choose which to merge)
+- [ ] More command line argument formats
+  - [x] Simple equal format: `{field}={value}`
+  - [ ] [getopt](https://en.wikipedia.org/wiki/Getopt) format: `-h --{field} {value}`
+- [ ] Various use cases
+  - [ ] Non-panic function
+  - [ ] Immutable function
+- [ ] Field Alias
+- [ ] Support complex field type
+- [ ] Document
+- [ ] Readme
+- [ ] ...

src/lib.rs

@@ -1,14 +1,54 @@
-pub fn add(left: usize, right: usize) -> usize {
-    left + right
-}
+extern crate proc_macro;
+
+use proc_macro::TokenStream;
+use quote::quote;
+use syn::{parse_macro_input, Data, DeriveInput, Fields};
 
-#[cfg(test)]
-mod tests {
-    use super::*;
+#[proc_macro_derive(MergeCfg)]
+pub fn merge_cfg_derive(input: TokenStream) -> TokenStream {
+    let ast = parse_macro_input!(input as DeriveInput);
+    let struct_ident = &ast.ident;
+
+    let mut merge_handle = quote! {};
+    match ast.data {
+        Data::Struct(s) => match s.fields {
+            Fields::Named(fields) => {
+                for field in &fields.named {
+                    let field_ident = field.ident.as_ref().unwrap();
+                    let field_name = field_ident.to_string();
+                    let field_type = field.ty.clone();
+                    // cast and set value
+                    merge_handle = quote! {
+                        #merge_handle
+                        #field_name => {
+                            self.#field_ident = kv[1].parse::<#field_type>().unwrap()
+                        }
+                    };
+                }
+            }
+            _ => panic!("MergeCfg can only be derived on structs with named fields"),
+        },
+        _ => panic!("MergeCfg can only be derived on structs"),
+    }
 
-    #[test]
-    fn it_works() {
-        let result = add(2, 2);
-        assert_eq!(result, 4);
+    // final expanded code
+    quote! {
+        impl #struct_ident {
+            pub fn merge_cfg(&mut self) {
+                let args: Vec<String> = ::std::env::args().collect();
+                    if args.len() > 1 {
+                        for arg in args.iter().skip(1) {
+                            let kv: Vec<&str> = arg.split('=').collect();
+                            if kv.len() == 2 {
+                                match kv[0] {
+                                    #merge_handle
+                                    _ => {}
+                                }
+                            }
+                        }
+                    }
+            }
+        }
     }
+    .into()
 }