blog: maybedoer (#153)

blog/maybedoer-2020-05-23.markdown

@@ -0,0 +1,167 @@
+---
+title: "maybedoer: the Maybe Monoid for Go"
+date: 2020-05-23
+tags:
+ - go
+ - golang
+ - monoid
+---
+
+# maybedoer: the Maybe Monoid for Go
+
+I recently posted (a variant of) this image of some Go source code to Twitter
+and it spawned some interesting conversations about what it does, how it works
+and why it needs to exist in the first place:
+
+<center>![the source code of package maybedoer](/static/blog/maybedoer.png)</center>
+
+This file is used to sequence functions that could fail together, allowing you
+to avoid doing an `if err != nil` check on every single fallible function call.
+There are two major usage patterns for it.
+
+The first one is the imperative pattern, where you call it like this:
+
+```go
+md := new(maybedoer.Impl)
+
+var data []byte
+
+md.Maybe(func(context.Context) error {
+  var err error
+ 
+  data, err = ioutil.ReadFile("/proc/cpuinfo")
+ 
+  return err
+})
+
+// add a few more maybe calls?
+
+if err := md.Error(); err != nil {
+  ln.Error(ctx, err, ln.Fmt("cannot munge data in /proc/cpuinfo"))
+}
+```
+
+The second one is the iterative pattern, where you call it like this:
+
+```go
+func gitPush(repoPath, branch, to string) maybedoer.Doer {
+  return func(ctx context.Context) error {
+    // the repoPath, branch and to variables are available here
+    return nil
+  }
+}
+
+func repush(ctx context.Context) error {
+  repoPath, err := ioutil.TempDir("", "")
+  if err != nil {
+    return fmt.Errorf("error making checkout: %v", err)
+  }
+
+  md := maybedoer.Impl{
+    Doers: []maybedoer.Doer{
+      gitConfig, // assume this is implemented
+      gitClone(repoPath, os.Getenv("HEROKU_APP_GIT_REPO")), // and this too
+      gitPush(repoPath, "master", os.Getenv("HEROKU_GIT_REMOTE")),
+    },
+  }
+  
+  err = md.Do(ctx)
+  if err != nil {
+    return fmt.Errorf("error repushing Heroku app: %v", err)
+  }
+  
+  return nil
+}
+```
+
+Both of these ways allow you to sequence fallible actions without having to
+write `if err != nil` after each of them, making this easily scale out to
+arbitrary numbers of steps. The design of this is inspired by a package used at
+a previous job where we used it to handle a lot of fiddly fallible actions that
+need to happen one after the other.
+
+However, this version differs because of the `Doers` element of
+`maybedoer.Impl`. This allows you to specify an entire process of steps as long
+as those steps don't return any values. This is very similar to how Haskell's
+[`Data.Monoid.First`](http://hackage.haskell.org/package/base-4.14.0.0/docs/Data-Monoid.html#t:First)
+type works, except in Go this is locked to the `error` type (due to the language
+not letting you describe things as precisely as you would need to get an analog
+to `Data.Monoid.First`). This is also similar to Rust's `and_then` combinator.
+
+If we could return values from these functions, this would make `maybedoer`
+closer to being a monad in the Haskell sense. However we can't so we are locked
+to one specific instance of a monoid. I would love to use this for a pointer (or
+pointer-like) reference to any particular bit of data, but `interface{}` doesn't
+allow this because `interface{}` matches _literally everything_:
+
+```go
+var foo = []interface{
+  1,
+  3.4,
+  "hi there",
+  context.Background(),
+  errors.New("this works too!"),
+}
+```
+
+This could mean that if we changed the type of a Doer to be:
+
+```go
+type Doer func(context.Context) interface{}
+```
+
+Then it would be difficult to know how to handle returns from the function.
+Arguably we could write some mechanism to check if it is an error:
+
+```go
+result := do(ctx)
+if result != nil {
+  switch result.(type) {
+  case error:
+    return result // result is of type error magically
+  default:
+    md.return = result
+  }
+}
+```
+
+But then it would be difficult to know how to pipe the result into the next
+function, unless we change Doer's type to be:
+
+```go
+type Doer func(context.Context, interface{}) interface{}
+```
+
+Which would require code that looks like this:
+
+```go
+func getNumber(ctx context.Context, _ interface{}) interface{} {
+  return 2
+}
+
+func double(ctx context.Context, num interface{}) interface{} {
+  switch num.(type) {
+  case int:
+    return 2+2
+  default:
+    return fmt.Errorf("wanted num to be an int, got: %T", num)
+  }
+  
+  return nil
+}
+```
+
+But this kind of repetition would be required for _every function_. I don't
+really know what the best way to solve this in a generic way would be, but I'm
+fairly sure that these fundamental limitations in Go prevent this package from
+being genericized to handle function outputs and inputs beyond what you can do
+with currying (and maybe clever pointer usage).
+
+I would love to be proven wrong though. If anyone can take this [source code
+under the MIT license](/static/blog/maybedoer.go) and prove me wrong, I will
+stand corrected and update this blogpost with the solution. 
+
+This kind of thing is more easy to solve in Rust with its
+[Result](https://doc.rust-lang.org/std/result/) type; and arguably this entire
+problem solved in the Go package is irrelevant in Rust because this solution is
+in the standard library of Rust.

static/blog/maybedoer.go

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+// Package maybedoer contains a pipeline of actions that might fail. If any action
+// in the chain fails, no further actions take place and the error becomes the pipeline
+// error.
+// 
+// MIT License
+package maybedoer
+
+import "context"
+
+// Doer is a function that implements a fallible action that can be done.
+type Doer func(context.Context) error
+
+// Impl sequences a set of actions to be performed via calls to
+// `Maybe` such that any previous error prevents new actions from being
+// performed.
+//
+// This is, conceptually, just a go-ification of the Maybe monoid, but
+// defined to the error type in Go.
+type Impl struct {
+	Doers []Doer
+	err   error
+}
+
+// Do executes the list of doers, right-folding the functions and seeing if one
+// returns an error. This is semantically identical to Data.Monoid.First in
+// Haskell, but specific to the error type in Go. Ideally this could be generalized
+// to any pointer-like datatype in Go, but Rob Pike says we can't have nice things.
+//
+// See the Haskell documentation for Data.Monad.First for more information:
+// http://hackage.haskell.org/package/base-4.14.0.0/docs/Data-Monoid.html#t:First
+func (c *Impl) Do(ctx context.Context) error {
+	for _, doer := range c.Doers {
+		c.Maybe(ctx, doer)
+		if c.err != nil {
+			return c.err
+		}
+	}
+
+	return nil
+}
+
+// Maybe performs `f` if no previous call to a Maybe'd action resulted
+// in an error
+func (c *Impl) Maybe(ctx context.Context, f func(ctx context.Context) error) {
+	if c.err == nil {
+		c.err = f(ctx)
+	}
+}
+
+// Error returns the first error encountered in the Error chain.
+func (c *Impl) Error() error {
+	return c.err
+}