maybedoer

Signed-off-by: Xe Iaso <me@christine.website>

maybedoer/maybedoer.go

@@ -0,0 +1,56 @@
+// 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.
+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 {
+		select {
+		case <-ctx.Done():
+			return ctx.Err()
+		default:
+			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
+}