Jeff C. Britton wrote:
I am trying to modify an example in RealWorldHaskell from Chapter 24. The example is the first code snippet labeled -- file: ch24/Compressor.hs
I am trying to replace the use of Readline with Haskeline. In my code the forkIO thread does not run. I guessed that since the result of the worker thread was thrown away that perhaps laziness was the problem. So, I attempted to use `seq`, but that does not work either.
The forkIO is not run because your code never actually runs it. :) The snippet let f = runWorker (worker path) in f `seq` do return f loop binds a value (here an `IO` action) to the variable `f`, then makes sure that the variable is evaluated to weak head normal form (which is something quite different from executing the `IO` action), and then combines the IO action `return f` (which has no side effects, but returns the value of `f`) with `loop`. The key point to understand here is that IO actions are first-class values: you can bind them to variables and combine them with `>>` and `>>=`. In a sense, you never execute IO actions, you only build them. The only time when something is executed is when the Haskell compiler executes the IO action assigned to the `main` variable. What you had in mind is a program that combines the IO action `f` directly with the `loop`, like this: let f = runWorker (worker path) in do f loop Best regards, Heinrich Apfelmus -- http://apfelmus.nfshost.com