One thing that annoyed me when I looked at Go years ago was that it seemed like strings where basically the only data type of unlimited size that was supported

One thing that annoyed me when I looked at Go years ago was that it seemed like strings where basically the only data type of unlimited size that was supported as map keys. I think that only types for which equality is defined in the language could be used as keys. Those types where numbers, characters and all other "small" fixed-size types, structs all of those members had equality defined, arrays of a type with equality, and strings. The size of an array is part of the type, so in that list of types with equality the only source of unlimited sizedness are strings. I guess that cover most cases where you want a type of unknown size as a key, but I found it pretty annoying. If you want to use, say, polynomials with integer coefficients as keys in a map, you'd have to either choose a fixed maximum size (degree, for example, or number of monomials) for the polynomials or convert back and forth between strings. It's not the only language that forces you to convert things to strings and back for use as map keys (AWK and Lua come to mind), but I wish it didn't have that limitation.