blog: add go interfaces considered harmful post

blog/go-interfaces-considered-harmful-2018-12-03.markdown

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+---
+title: Go Interfaces Considered Harmful
+date: 2018-12-03
+---
+
+# Go Interfaces Considered Harmful
+
+A group of blind men heard that a strange animal had been brought to the town function, but none of them were aware of its type.
+
+```
+package blindmen
+
+type Animal interface{}
+
+func Town(strangeAnimal Animal) {
+```
+
+Out of curiosity, they said: “We must inspect and know it by type switches and touch, of which we are capable”.
+
+```
+type Toucher interface {
+  Touch() interface{}
+}
+```
+
+So, they sought it out, and when they found it they groped about it.
+
+```
+for man := range make([]struct{}, 6) {
+   go grope(man, strangeAnimal.(Toucher).Touch())
+}
+```
+
+In the case of the first person, whose hand landed on the trunk, said “This being is like a thick snake”.
+
+```
+type Snaker interface {
+  Snake()
+}
+
+func grope(id int, thing interface{}) {
+  switch thing.(type) {
+  case Trunker:
+    log.Printf("man %d: this thing is like a thick snake", id)
+```
+
+For another one whose hand reached its ear, it seemed like a kind of fan.
+
+```
+type Fanner interface {
+  Fan()
+}
+
+// in grope switch block
+case Fanner:
+  log.Printf("man %d: this thing is like a kind of fan", id)
+```
+
+As for another person, whose hand was upon its leg, said, the it is a pillar like a tree-trunk.
+
+```
+type TreeTrunker interface {
+  TreeTrunk()
+}
+
+// in grope switch block
+case TreeTrunker:
+  log.Printf("man %d: this thing is like a tree trunk", id)
+```
+
+The blind man who placed his hand upon its side said, “it is a wall”.
+
+```
+type Waller interface {
+  Wall()
+}
+
+// in grope switch block
+case Waller:
+  log.Printf("man %d: this thing is like a wall", id)
+```
+
+Another who felt its tail, described it as a rope.
+
+```
+type Roper interface {
+  Rope()
+}
+
+// in grope switch block
+case Roper:
+  log.Printf("man %d: this thing is like a rope", id)
+```
+
+The last felt its tusk, stating the thing is that which is hard, smooth and like a spear.
+
+```
+type Tusker interface {
+  Tusk()
+}
+
+// in grope switch block
+case Tusker:
+  log.Printf("man %d: this thing is hard, smooth and like a spear", id)
+```
+
+All of the men spoke fact about the thing, but none of them spoke the truth of what it was.
+
+```
+// after grope switch block
+log.Printf("%T", thing) // prints Elephant
+```