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course/12-local_development/exercises/11-custom_package/readme.md

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+# Custom Package
+
+Let's write a package to import and use in `hellogo`.
+
+Create a sibling directory at the same level as the `hellogo` directory:
+
+```bash
+mkdir mystrings
+cd mystrings
+```
+
+Initialize a module:
+
+```bash
+go mod init {REMOTE}/{USERNAME}/mystrings
+```
+
+Then create a new file `mystrings.go` in that directory and paste the following code:
+
+```go
+// by convention, we name our package the same as the directory
+package mystrings
+
+// Reverse reverses a string left to right
+// Notice that we need to capitalize the first letter of the function
+// If we don't then we won't be able access this function outside of the
+// mystrings package
+func Reverse(s string) string {
+  result := ""
+  for _, v := range s {
+    result = string(v) + result
+  }
+  return result
+}
+```
+
+Note that there is no `main.go` or `func main()` in this package.
+
+`go build` won't build an executable from a library package. However, `go build` will still compile the package and save it to our local build cache. It's useful for checking for compile errors.
+
+Run:
+
+```bash
+go build
+```