move: advanced funcs to functional programming

27-functional-programming/log-parser-exp/report.go

@@ -0,0 +1,85 @@
+package main
+
+import "io"
+
+type (
+	parserFunc func(io.Reader) ([]result, error)
+	filterFunc func(result) bool
+	groupFunc  func(result) string
+	outputFunc func(io.Writer, []result) error
+)
+
+type report struct {
+	input     io.Reader
+	output    io.Writer
+	parser    parserFunc
+	filterer  filterFunc
+	grouper   groupFunc
+	outputter outputFunc
+}
+
+func newReport() *report {
+	return &report{
+		// parser:   textParser,
+		filterer: noopFilter,
+	}
+}
+
+func (r *report) from(reader io.Reader) *report {
+	r.input = reader
+	return r
+}
+
+func (r *report) to(writer io.Writer) *report {
+	r.output = writer
+	return r
+}
+
+func (r *report) retrieveFrom(fn parserFunc) *report {
+	r.parser = fn
+	return r
+}
+
+func (r *report) filterBy(fn filterFunc) *report {
+	r.filterer = fn
+	return r
+}
+
+func (r *report) groupBy(fn groupFunc) *report {
+	r.grouper = fn
+	return r
+}
+
+func (r *report) writeTo(fn outputFunc) *report {
+	r.outputter = fn
+	return r
+}
+
+func (r *report) run() ([]result, error) {
+	if r.parser == nil {
+		panic("report retriever cannot be nil")
+	}
+
+	results, err := r.parser(r.input)
+	if err != nil {
+		return nil, err
+	}
+
+	// noop if filterer is nil
+	results = filterBy(results, r.filterer)
+
+	// grouper is more tricky
+	// you don't want to create an unnecessary map
+	if r.grouper != nil {
+		results = groupBy(results, r.grouper)
+	}
+
+	// prefer: noop output
+	if r.output != nil {
+		if err := r.outputter(r.output, results); err != nil {
+			return nil, err
+		}
+	}
+
+	return results, nil
+}