Query isn't a String. String interpolation[^1] would de-sugar to something like this:

  query conn ("select x from users where name like " ++ username)

++ is a function that expects two Strings. The "select..." stuff isn't a String, quotation marks not withstanding. When we try to hand a Query to ++, the compiler screams bloody murder.

Longer explanation: I suspect the syntax is a bit confusing, since while I keep saying "select ..." is a Query, it looks an awful lot like a String. Here's what's going on. Haskell has a typeclass called IsString. Query is an instance of IsString, as is String.[^2]

Quoted text can represent any instance of IsString. So the compiler sees a function that expects a Query and an IsString of some sort, and through the magic of type inference, it decides that the IsString must be a Query.[^3] And when you try to use a function that concatenates Strings on that Query, it knows that Something's Not Right.

[1]: Haskell doesn't have string interpolation. But if it did, this is how it would work.

[2]: And other instances as well. postgresql-simple actually uses ByteStrings, not Strings, for performance.

[3]: I've fuzzed the evaluation order a bit, for simplicity. In practice the first error reported might be that you've passed 2 arguments to a function that expects 3.