Variable Naming – How and Why to Use Underscores

You are apparently misunderstanding the reason prefix underscores are bad practice. To make it short, it is because the C and C++ standard reserve these prefix for implementation details, for example for standard library implementation. (note that _ and __ are not reserved for the same things, see comments)

Even if the names are under scope (namespace, class, etc.), there can be some global names, in particular macros, which use these prefix and might silently break your code if you use them too.

So, basically, most of the time it is safe to use these prefix BUT if you don't use them you have a 100% guarantee that your naming will never conflict with the implementation names.

Which is why, in doubt, don't use these prefix.

I suspect it's done as quasiquoting because this will interpolate the string at compile time, and due to the pure nature of functions in Haskell this is possible while other languages lack such ability because of their impure nature.

Think about it, other languages have to do it during runtime each time which is going to be much slower than the compile time once and done quasi-quoted interpolation.

If you want to do it at runtime explicitly then you should be able to do some simple abstractions that take advantage of Haskell's strings being lists, or you could create a different data structure that's similar pretty easily - but I would go for the more efficient quasi-quoted approach if I were doing this in any code I actually cared about.

That said, I found on Hackage the Data.Text.Format package which appears to do C# style substitution pretty straight forward, here's an answer showing an example on SO: String formatting in Haskell - though as noted by Bryan O'Sullivan and others on that question that answer usess an out-dated package and there's a newer one Bryan has created (my guess the Data.Text.Format package is his new one that should be used) so the usage may be slightly different from in that SO example, though appears fairly similar.