[Haskell-cafe] Execution order in IO

[Tom Ellis]

Hi Jon,

On Wed, Apr 15, 2015 at 10:07:24AM +0100, Jon Schneider wrote:
> With lazy evaluation where is it written that if you write things with no
> dependencies with a "do" things will be done in order ? Or isn't it ?

I'm not sure where this is written, but it's certainly a property of the IO
type.  In the expression

    do
       x1 <- action1
       x2 <- action2
       ...

then the IO action of the expression `action1` will occur before that of
`action2`.

(As a caveat, one has to be careful about the concept of "when an action
occurs".  If `action1` involved reading a file lazily with
`System.IO.readFile`[1], say, then the actual read may not take place until
`action2` has already finished.  However, from the point of view behaviour
we consider "observable", a lazy read is indistinguishable from a strict
read.  Lazy IO is rather counterintuitive.  I suggest you stay away from
it!)

As a side point, appeals to the "real world" in attempts to explain this are
probably unhelpful at best.  GHC may well implement IO using a fake value of
type `RealWorld` but that's beside the point.  A conforming Haskell
implementation is free to implement IO however it sees fit.

> Is it a feature of the language we're supposed to accept ?

Sort of.  It's a property of the IO type.

> Is it something in the implementation of IO ?

Yes.

> Is the do keyword more than just a syntactic sugar for a string of binds
> and lambdas ?

No.

Tom

[1] http://hackage.haskell.org/package/base-4.8.0.0/docs/System-IO.html#v:readFile