Construct a whole program of functions only with say `readonly` (in c#) or `const` (in EcmaScript) or only `:=` (in Go) "variable" "assignment-declarations", ne

Construct a whole program of functions only with say `readonly` (in c#) or `const` (in EcmaScript) or only `:=` (in Go) "variable" "assignment-declarations", never mutating them after initial declaration. It's possible and it's one of the core tenets of purely FP.