[lmm]

It's in a do block, so you can see that these are not simple function calls; they're being composed via some monad. In cases where you're actually chaining values together it's more obvious which things are which:

    do
      value1 <- function1 --effectful function
      let value2 = function2(value1) --pure function
      value3 <- function3(value1, value2) --effectful function
      ...

But the notation is a bit more magic for this "no return" case; I prefer the Scala approach where even if you don't care about the return values you'd have to write this as

    for {
      _ ← cd "/tmp" // effectful function
      _ ← mkdir "test" //effectful function
      _ = someCalculation() //pure function
      ...

[Peaker]

If you enable -Wall, Haskell forces you to use "_ <-" for any action that has a stateful result.

But the "let" vs no "let" is a pretty strong hint anyway :)

[mercurial]

Control.Monad.void is your friend.

[Peaker]

Data.Functor.void you mean? :)