[Haskell-cafe] lambda calculus and equational logic

Peter Gammie peteg42 at gmail.com
Tue Jul 13 10:15:55 EDT 2010

Ben, comments from the peanut gallery:

On 13/07/2010, at 11:51 PM, Ben Lippmeier wrote:

> What kind of equality do you use for  getChar :: IO Char ?

Surely this is easy: getChar denotes a particular IO action, which is always the same thing (i.e. self-identical and distinct from all other IO actions).

> By "massively complicated" you mean "harder than the simplest case." Haskell's do-notation makes the "state of the world" implicit, and performing the desugaring makes it explicit again -- but then that state isn't really "the state of the word"...

... which tells you that the passing-the-world semantics is not a faithful model of I/O.

In the archives of this list there are many comments on exactly this topic. A couple of quick pointers:

- Wouter Swierstra wrote a paper on this.


- Resumptions are a better model:


Intuitively the state monad used by GHC works because it only needs to track dependencies between I/O actions - there is no need for the compiler to impose a total order on everything going on in the world (taken to be all of the context that could influence the execution of the program). For interesting programs it cannot, anyway.



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