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:readFi...