Presumably animats is talking about things like "do notation" in Haskell, not innocuous cases of function composition or first-class functions in imperative languages.
I read it as being about Scala and similar languages, where you can not freely rewrite function compositions because they may have side effects.
It may not be big problem from the programmer's point of view. But the compiler surely suffers because of that.
Interestingly, on Haskell-land the debate is about merging "do notation" and normal notation. On practice the difference is limiting, and arguably the types alone are enough to represent the difference.
I don't know, it's far from perfect, but is there a better way to do sequencing in a pure functional language? You have to be able to specify order of execution to tackle real world tasks unless you do callback/continuation passing style right? I find that a lot less intuitive for most applications.
Do notation isn't about sequencing, it's syntactic sugar for the monadic 'bind' operator. Sequencing happens because of data dependencies. This is part of the confusing story about monads -- the fact that every monad is a monoid doesn't imply a straightforward linear sequencing, as something like the tardis monad illustrates.
More constructively, it could be argued that IO just does not fall in the remit of functional programming. SPJ doesn't refer to Peter Landin much, but Landin's perspective is valuable. In 1964 he wrote "Is there some way of extending the notion of [arithmetic expressions] so as to serve some of the needs of computer users without all the elaborations of using computers?" (The Mechanical Evaluation of Expressions).
IMO this is exactly what the Haskell tradition is pursuing. Landin, quite modestly, doesn't anticipate that entire computer systems will be expressible in functional form. Implicit data structures are just one example of a mechanism that is incompatible with the idea that everything is an expression whose internal representation is managed by the language runtime (itself presumably involving mutable data).
I don't think Landin was experiencing a failure of vision--I think he saw clearly that some of the "elaborations" of computing are remote from a functional model.
Fair enough; I knew do notation was sugar for bind, but I didn't realize/forgot about the time traveling monad.
Follow up questions bc I'm still a Haskell noob:
Do you know if async/wait from Control.Concurrent.Async ensure linear sequencing?
Could the sequencing problem be solved if linear types make their way into Haskell? (They seem to mainly be about memory management, but I'm not sure about other potential applications)
