Go, gofmt, and diffs

2015-06-02

Gofmt is a Go program formatter. Its universal adoption by the Go community has resulted in a consistent formatting style among most if not all Go source code.

When making changes to Go code, gofmt sometimes causes changes to lines of code that weren’t originally modified. This can done for example to keep indentation consistent. This can make diffs hard to read.

For example, the following is a diff of a simple type declaration:

$ git diff

diff --git a/main.go b/main.go
index 2c7af41..d9e59cd 100644
--- a/main.go
+++ b/main.go
@@ -1,13 +1,12 @@
 type foo struct {
-    jabberwocky  int
-    quux         complex128
-    galumphing   string
-    vorpal       float64
-    bandersnatch float64
-    mimsy        struct {
-        beaming  int
-        tumtummy string
-        brillig  float64
+    jabberwocky int
+    quux        complex64
+    tumtummy    string
+    galumphing  string
+    vorpal      float64
+    mimsy       struct {
+        beaming int
+        brillig float64
	 }
 }

Because gofmt has rewritten the source code to have consistent indentation, at least a whitespace change has been made to every line. It can be hard to see what the real change is.

To see the real changes, the diff algorithm should be configured to ignore whitespace. The diff below makes it quite obvious what the actual change is:

$ git diff -b

diff --git a/main.go b/main.go
index 2c7af41..d9e59cd 100644
--- a/main.go
+++ b/main.go
@@ -1,12 +1,11 @@
 type foo struct {
	 jabberwocky int
-    quux         complex128
+    quux        complex64
+    tumtummy    string
	 galumphing  string
	 vorpal      float64
-    bandersnatch float64
	 mimsy       struct {
		 beaming int
-        tumtummy string
		 brillig float64
	 }
 }

It’s now quite clear that quux has had its type modified, bandersnatch has been removed, and tumtummy has been moved from the inner struct, to the outer struct.

This type of diff can typically be created by passing the --ignore-space-change flag (the short version is -b). This works with Git, Mercurial, Subversion, and well as the diff command that comes with diffutils. GUI based diffs such as those produced by Github will typically have an ignore-whitespace option as well.