getOrMake, first pass

Author: lostluck

content/posts/2025-01-08-getOrMake.md

@@ -0,0 +1,174 @@
+---
+title: "Tutorial: GetOrMake for maps."
+slug: tutorial-getormake
+date: 2025-01-08T22:06:58-08:00
+tags: 
+ - golang
+ - tutorial
+categories: 
+ - Go
+ - Tutorial
+---
+
+Today I'm going to quickly go over one of my favourite convenience functions,
+made possible with Go's Generics. It's invaluable if you deal with maps of maps
+in Go.
+
+<!--more-->
+
+### Maps
+
+Maps are one of the the original "generic" types available to Go users. 
+If you aren't familiar with them, you can learn about them in [Effective Go](https://go.dev/doc/effective_go#maps).
+
+The important bit is that the key type must be `comparable`, which is a special
+built in interface, that defines all comparable types.
+See https://pkg.go.dev/builtin#comparable for more details on that.
+
+
+### Nested Maps
+
+However, the values of those maps can be any Go type at all, including any other map.
+
+```go
+// nested maps, two deep.
+var mapOfMaps map[string]maps[string]string
+```
+
+And the values for those *nested* maps, can also have maps.
+
+```go
+// nested maps, two deep.
+var mapOfMapsOfMaps map[string]maps[string]map[string]string
+```
+
+### Setting nested maps
+
+But this nesting has a downside. What if you want to get to that final map, so
+you can set the value on it's key.
+
+Say it's a field Map on a struct, and we have a method to set the inner most value.
+The code would then need to initialize all the maps along the way to setting the
+value.
+
+```go
+func (w *mWrapper) Set(k1,k2,k3, value string) {
+    if w.Map == nil {
+        w.Map = make(map[string]maps[string]map[string]string)
+    }
+    v1, ok := w.Map[k1]
+    if !ok {
+        v1 = make(map[string]maps[string]string)
+        w.Map[k1] = v1
+    }
+    v2, ok := v1[k2]
+    if !ok {
+        v2 = make(map[string]string)
+        v1[k2] = v2
+    }
+    v2[k3] = value
+} 
+```
+
+What's happening here, is that for each level with an inner map, we need to
+check if that map exists. We use the "comma ok" idiom, where the value of OK
+indicates if the map as a value for the given key or not. If it doesn't exist,
+we make a new instance of the map type, and assign it both to the value variable,
+and to the key in the map itself.
+
+As you can imagine, the more layers of nesting you have, the more tedious
+creating the earlier nested map types become.
+
+### Generics to the rescue!
+
+Using generics, we can avoid some of this!
+
+```go
+func (w *mWrapper) Set(k1,k2,k3, value string) {
+    if w.Map == nil {
+        w.Map = make(map[string]maps[string]map[string]string)
+    }
+    v1 := getOrMake(w.Map, k1)
+    v2 := getOrMake(v1, k2)
+    v2[k3] = value
+} 
+```
+
+Now it's eight lines shorter! 4 per level we've removed, including the tedious
+repetition of the nested type.
+
+Here's the code for `getOrMake`.
+
+```go
+
+func 
+
+// getOrMake is a generic helper function for extracting or initializing a sub map.
+func getOrMake[K, VK comparable, VV any, V map[VK]VV, M map[K]V](m M, key K) V {
+	v, ok := m[key]
+	if !ok {
+		v = make(V)
+		m[key] = v
+	}
+	return v
+}
+```
+
+First we've declared our generic types. 
+
+* K is the key for the outermost map.
+* VK is the key for the value map.
+* VV is the value for the value map,
+* V is the type of the value map: `map[VK]VV`
+* M the type of the map being passed in, `map[K]V`
+
+The function then takes in an instance `m` of type `M`, and the `key` of type `K`,
+returning an instance of the the value map.
+
+The then the code is pretty straightforward.
+
+We lookup the value of the key, and whether it exists or not. If the key has
+no value, then we do as we did before: create an instance of the value map, and
+assign that to the key. Then we return the value itself.
+
+I wouldn't pull out `getOrMake` the first time I need it, and it only works for
+maps. But if I start repeating the pattern around in a package, or I start to
+play with the inner types, using `getOrMake` can reduce some effort around that
+refactoring.
+
+### Aside: Top Level Maps
+
+It also can't work for top level maps. For example.
+
+```go
+func NoArgMakeMap[K comparable, V any, M map[K]V]() M {
+    return make(M)
+}
+```
+
+That doesn't work, since Go's type inference doesn't pull from the return value
+even if it would be helpful. 
+
+The closest you can get is to explicitly hint the type, by including an unused
+parameter.
+
+```go
+func OneArgMakeMap[K comparable, V any, M map[K]V](_ M) M {
+    return make(M)
+}
+```
+
+That'll be enough to have the compiler do the right thing. 
+
+There is at least [one](https://github.com/golang/go/issues/50285) issue in the
+Go issue tracker for this feature request, but no real proposals at this time.
+
+I don't think it's critical for Go to get this though, since the work around
+isn't terrible at this point.
+
+### Conclusion
+
+Generics in Go are a powerful tool to reduce boiler plate, even with small
+utility functions. Especially with Go's original generic types, like `map`!
+
+Thanks for reading.