feat: force function definition arguments and returns on a single line

README.md

@@ -292,6 +292,23 @@ This conversion only applies to structs defined in the same file. External struc
 
 ### Functions
 
+**Function signatures** are always collapsed to a single line:
+
+```go
+// Before — multi-line signature
+func Process(
+    ctx context.Context,
+    input Input,
+) (Output, error) {
+
+// After — single line
+func Process(ctx context.Context, input Input) (Output, error) {
+```
+
+This applies to function definitions, methods, function types, and interface methods.
+
+---
+
 **Standalone functions sorting:**
 1. Exported first, then unexported
 2. Within each group: by architectural layer (high-level first)

pkg/formatter/file.go

@@ -55,6 +55,7 @@ func FormatFile(filePath string, opts Options) error {
 		return nil
 	}
 
+	collapseFuncSignatures(f)
 	originalFieldOrder := collectOriginalFieldOrder(f)
 	convertPositionalToKeyed(f, originalFieldOrder)
 	reorderStructFields(f)

pkg/formatter/spacing.go

@@ -127,3 +127,58 @@ func normalizeSpacing(f *dst.File) {
 		return true
 	})
 }
+
+func collapseFuncSignatures(f *dst.File) {
+	dst.Inspect(f, func(n dst.Node) bool {
+		switch node := n.(type) {
+		case *dst.FuncDecl:
+			if node.Type != nil {
+				collapseFuncType(node.Type)
+			}
+		case *dst.FuncLit:
+			if node.Type != nil {
+				collapseFuncType(node.Type)
+			}
+		case *dst.TypeSpec:
+			if ft, ok := node.Type.(*dst.FuncType); ok {
+				collapseFuncType(ft)
+			}
+		case *dst.InterfaceType:
+			if node.Methods != nil {
+				for _, method := range node.Methods.List {
+					if ft, ok := method.Type.(*dst.FuncType); ok {
+						collapseFuncType(ft)
+					}
+				}
+			}
+		}
+
+		return true
+	})
+}
+
+func collapseFuncType(ft *dst.FuncType) {
+	if ft.Params != nil {
+		collapseFieldList(ft.Params)
+	}
+	if ft.Results != nil {
+		collapseFieldList(ft.Results)
+	}
+}
+
+func collapseFieldList(fl *dst.FieldList) {
+	// Remove newlines from opening paren
+	fl.Decs.Opening = nil
+
+	for i, field := range fl.List {
+		// First field: no newline before
+		if i == 0 {
+			field.Decs.Before = dst.None
+		} else {
+			// Subsequent fields: space only (comma handled automatically)
+			field.Decs.Before = dst.None
+		}
+		// No newline after any field
+		field.Decs.After = dst.None
+	}
+}

pkg/formatter/testdata/expected.go

@@ -1,7 +1,9 @@
 package main
 
 import (
+	"context"
 	"fmt"
+	"net/http"
 	"os"
 	"strings"
 )
@@ -66,6 +68,9 @@ type StatusCode MyString
 
 type Priority int
 
+// Test: function type should collapse
+type MultiLineHandler func(w http.ResponseWriter, r *http.Request)
+
 type Reader interface {
 	Read(p []byte) (n int, err error)
 }
@@ -78,6 +83,11 @@ type Closer interface {
 	Close() error
 }
 
+// Test: interface method should collapse
+type MultiLineInterface interface {
+	Process(ctx context.Context, input Input) (Output, error)
+}
+
 type ReadWriter interface {
 	Read(p []byte) (n int, err error)
 	Write(p []byte) (n int, err error)
@@ -118,6 +128,11 @@ func (s *Server) PublicMethod() {}
 
 func (s *Server) handleRequest() {}
 
+// Test: multi-line method signature should collapse
+func (s *Server) multiLineMethod(ctx context.Context, input string) (string, error) {
+	return input, nil
+}
+
 // Test: method declared before its type
 func (s *Server) privateMethod() {
 	return
@@ -227,6 +242,11 @@ type WithEmbedded struct {
 	Extra string
 }
 
+// Types for interface test
+type Input struct{}
+
+type Output struct{}
+
 func HelperUpper() {}
 
 func ProcessDataPublic(data string) string {
@@ -360,6 +380,16 @@ func helperLower() {
 	fmt.Println("helper")
 }
 
+// Test: multi-line func signature should collapse to single line
+func multiLineFunc(a int, b string, c bool) error {
+	return nil
+}
+
+// Test: multi-line return values should collapse
+func multiLineReturns() (result string, err error) {
+	return "", nil
+}
+
 func processData(data string) string {
 	return strings.ToUpper(data)
 }

pkg/formatter/testdata/input.go

@@ -1,7 +1,9 @@
 package main
 
 import (
+	"context"
 	"fmt"
+	"net/http"
 	"os"
 	"strings"
 )
@@ -338,3 +340,46 @@ func createKeyed() *PositionalTest {
 func createEmpty() *PositionalTest {
 	return &PositionalTest{}
 }
+
+// Test: multi-line func signature should collapse to single line
+func multiLineFunc(
+	a int,
+	b string,
+	c bool,
+) error {
+	return nil
+}
+
+// Test: multi-line method signature should collapse
+func (s *Server) multiLineMethod(
+	ctx context.Context,
+	input string,
+) (string, error) {
+	return input, nil
+}
+
+// Test: multi-line return values should collapse
+func multiLineReturns() (
+	result string,
+	err error,
+) {
+	return "", nil
+}
+
+// Test: function type should collapse
+type MultiLineHandler func(
+	w http.ResponseWriter,
+	r *http.Request,
+)
+
+// Test: interface method should collapse
+type MultiLineInterface interface {
+	Process(
+		ctx context.Context,
+		input Input,
+	) (Output, error)
+}
+
+// Types for interface test
+type Input struct{}
+type Output struct{}