[Roboprog]

It seems to me that yes, you need an "escape hatch" in an FP language to make your updates.

It would be nice if the language required such functions, and the modules in which they reside, to be flagged. (I don't know if Haskell does something like this with mutation, or not)

It also seems that an "actor model" would be a good way to encapsulate the updates in an otherwise FP program by having a loop/reduce/fold wrapped around the mutable data responding to request-events and generating responses. This allows the other pure/immutable/idempotent type of code to remain isolated from it.

[dottedmag]

Flagged like "State T" in function type, which Haskell has had since ~1995?

[tome]

Or IO, or ST, or STM, or any other number of Haskell features that Roboprog has correctly predicted to exist.

[dllthomas]

Performance-wise, StateT is not an escape-hatch. It can be great for removing boilerplate, but it's simply another way of writing (s -> (a, s)).

IO, ST, and STM mentioned by tome better match the description of "escape hatch" - though for ST and STM they are very carefully shaped escape hatches.