fn: add fundamental functional primitives

fn/fn.go

@@ -0,0 +1,30 @@
+package fn
+
+// Unit is a type alias for the empty struct to make it a bit less noisy to
+// communicate the informationaless type.
+type Unit = struct{}
+
+// Comp is left to right function composition. Comp(f, g)(x) == g(f(x)). This
+// can make it easier to create on the fly closures that we may use as
+// arguments to other functions defined in this package (or otherwise).
+func Comp[A, B, C any](f func(A) B, g func(B) C) func(A) C {
+	return func(a A) C {
+		return g(f(a))
+	}
+}
+
+// Iden is the left and right identity of Comp. It is a function that simply
+// returns its argument. The utility of this function is only apparent in
+// conjunction with other functions in this package.
+func Iden[A any](a A) A {
+	return a
+}
+
+// Const is a function that accepts an argument and returns a function that
+// always returns that value irrespective of the returned function's argument.
+// This is also quite useful in conjunction with higher order functions.
+func Const[A, B any](a A) func(B) A {
+	return func(_ B) A {
+		return a
+	}
+}