blog: maybedoer (#153)

2 files changed, 53 insertions, 0 deletions

diff --git a/static/blog/maybedoer.go b/static/blog/maybedoer.go
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+++ b/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
+}
diff --git a/static/blog/maybedoer.png b/static/blog/maybedoer.png
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+++ b/static/blog/maybedoer.png