Write README

Author: sublee

README.md

@@ -1,12 +1,15 @@
 # Convgen
 
-![Convgen Logo](assets/convgen.jpg)]
+<p align="center">
+<img src="assets/convgen.jpg" alt="Convgen Logo" width="320" />
+</p>
 
 Convgen generates **type-to-type conversion code** for Go with **type-safe
 configuration** and **detailed diagnostics**.
 
 ```go
 //go:build convgen
+
 var EncodeUser = convgen.Struct[User, api.User](nil,
     convgen.RenameReplace("", "", "Id", "ID"), // Replace Id with ID in output types before matching
     convgen.Match(User{}.Name, api.User{}.Username), // Explicit field matching
@@ -17,13 +20,42 @@ var EncodeUser = convgen.Struct[User, api.User](nil,
 
 - **Struct, Union, and Enum conversions**  
   Automatically matches fields, implementations, and members by name.
+
+  ```go
+  convgen.Struct[User, api.User]      // func(User) api.User
+  convgen.StructErr[User, api.User]   // func(User) (api.User, error)
+  convgen.Union[Job, api.Job]         // func(Job) api.Job -- type UploadJob, type OrderJob, ...
+  convgen.UnionErr[Job, api.Job]      // func(Job) (api.Job, error)
+  convgen.Enum[Status, api.Status]    // func(Status) api.Status -- const StatusTodo, const StatusPending, ...
+  convgen.EnumErr[Status, api.Status] // func(Status) (api.Status, error)
+  ```
+
 - **Type-safe configuration**  
   All options are validated at compile time — no reflection, tags, string, or
   comment-based directives.
+
+  ```go
+  // Custom conversion functions must be reachable.
+  convgen.ImportFunc(strconv.Itoa)
+
+  // If User{}.Name is renamed by a refactoring tool,
+  // this directive will be updated accordingly.
+  convgen.Match(User{}.Name, api.User{}.Username)
+  ```
+  
 - **Detailed diagnostics**  
   *All* matching and conversion errors in a single pass are reported together,
   so you can fix everything at once instead of stopping at the first error.
 
+  ```
+  main.go:10:10: invalid match between User and api.User
+      ok:   Name    -> Username // forced at main.go:12:2
+      ok:   ID      -> ID [Id]
+      ok:   GroupID -> GroupID [GroupId]
+      FAIL: ?       -> Email // missing
+      FAIL: EMail   -> ?     // missing
+  ```
+
 ## Motivation
 
 Convgen is inspired by both [goverter](https://github.com/jmattheis/goverter)
@@ -36,104 +68,47 @@ injection. Convgen combines the best of both worlds, bringing **type-safe
 configuration** and **comprehensive diagnostics** to
 **type conversion code generation**.
 
-## Installation
-
-```bash
-go install github.com/sublee/convgen
-```
-
 ## Quick Start
 
-1. Add a build constraint to files containing Convgen directives:
-
-```go
-//go:build convgen
-```
-
-2. Declare your conversions:
-
-```go
-//go:build convgen
-package main
-
-import "github.com/sublee/convgen"
-
-// Simple struct conversion
-var EncodeUser = convgen.Struct[User, api.User](nil)
-```
-
-3. Run the generator:
-
-```bash
-go run github.com/sublee/convgen/cmd/convgen ./...
-```
-
-This generates `convgen_gen.go` with the implementation:
-
-```go
-func EncodeUser(in User) (out api.User) {
-    out.Name = in.Name
-    out.Email = in.Email
-    return
-}
-```
-
-## Example
+1. Install Convgen:
 
-```go
-//go:build convgen
+    ```bash
+    go install github.com/sublee/convgen
+    ```
 
-package main
+2. Add a build constraint to files containing Convgen directives:
 
-import "github.com/sublee/convgen"
+    ```go
+    //go:build convgen
+    ```
 
-// Create a module with shared configuration
-var mod = convgen.Module(
-    convgen.RenameToLower(true, true),
-)
+3. Declare your conversions:
 
-// Declare conversions
-var (
-    EncodeUser = convgen.Struct[User, api.User](mod,
-        convgen.Match(User{}.ID, api.User{}.Id), // explicit field matching
-    )
-    EncodeRole = convgen.Enum[Role, api.Role](mod,
-        api.ROLE_UNSPECIFIED, // default value
-        convgen.RenameTrimCommonPrefix(true, true),
-    )
-)
-```
+    ```go
+    var EncodeUser = convgen.Struct[User, api.User](nil)
+    ```
 
-## Configuration
+4. Run the generator:
 
-When field mappings are ambiguous, Convgen provides detailed diagnostics:
+    ```bash
+    convgen ./...
+    ```
 
-```
-main.go:10:10: invalid match between User and api.User
-    FAIL: ID -> ?  // missing
-    FAIL: ?  -> Id // missing
-```
+5. Convgen generates a `convgen_gen.go` file by copying your `//go:build convgen`
+   files and rewriting Convgen directives:
 
-Resolve with renaming rules:
-
-```go
-var EncodeUser = convgen.Struct[User, api.User](nil,
-    convgen.RenameToLower(true, true),
-)
-```
-
-Or explicit matching:
-
-```go
-var EncodeUser = convgen.Struct[User, api.User](nil,
-    convgen.Match(User{}.ID, api.User{}.Id),
-)
-```
+    ```go
+    func EncodeUser(in User) (out api.User) {
+        out.Name = in.Name
+        out.Email = in.Email
+        return
+    }
+    ```
 
 ## License
 
-MIT License - see [LICENSE](LICENSE) for details.
+MIT License -- see [LICENSE](LICENSE) for details.
 
 ## Author
 
-Heungsub Lee
+[Heungsub Lee](https://subl.ee/) and ChatGPT