Control.Monad.State.Strict.modify is lazy

[joeyh]

I've had programs that repeatedly modified state leak memory because modify does not force the f s thunk.

This is surprising in a library with "strict" in its name..

[ekmett]

Strict does not refer to the state itself, but rather the tuple, which has two different semantics it is forced to choose between, because lazy state can let more programs terminate than the strict state monad.

You can put $! whatever to get strict assignment.

[ekmett]

Changing the semantics of strict state to include forcing the thunk is out of scope.

That said,

1.) It would be a perfectly reasonable request to see about adding a strict-in-the-value State and Writer type to transformers. In particular with Writer there is no recourse by which you can force what you tell in a useful way.

2.) We do have an open issue (#2) to improve the documentation on State.Strict and Writer.Strict to summarize the issue better, to help mitigate confusion.