evaluate to lazy?

Andreas Marth Andreas-Haskell at gmx.net
Wed Nov 22 08:23:05 EST 2006

I know that 1:error "emsg" is not _|_. What is surprising for me is that
evaluate ( 1:error "emsg") does not raise an exception.
Therefore I have 2 points:

1.) I think evaluate should have more explanation in the library
documentation. (I read Alastair Reid: "Handling Exceptions in Haskell" and
Simon PEYTON JONES: "Tackling the Awkward Squad" and still missed this
point.) I think the following from "Tackling the Awkward Squad" (p. 41) is a
good candidate:
"a1, a2, a3, a4 :: IO ()
a1 = do { x <- evaluate 4; print x }
a2 = do { evaluate (head []); print "no" }
a3 = do { return (head []); print "yes" }
a4 = do { xs <- evaluate [1 `div` 0]; print (length xs) }
The first simply evaluates 4, binds it to x, and prints it; we could equally
well have written (return 4) instead. The second evaluates (head []), finds
an exceptional value, and throws an exception in the IO monad; the following
print never executes. In contrast a3 instead returns the exceptional value,
ignores it, and prints yes. Lastly, a4 evaluates the list [1 'div' 0], binds
it to xs, takes its length, and prints the result. The list contains an
exceptional value, but evaluate only evalutes the top level of its argument,
and does not look inside its recursive tructure"

2.) Is it useful that evaluate is not 'really strict' (does not fully
evaluate its argument)?
If I use
a5 = do {xs <- return [1 `div` 0]; print (length xs)}
 it is the same as a4. The exceptional value within the list is never
evaluated and so the print succeeds.
So why did I use evaluate [1 `div` 0] in the first place?
The same is true with my excample with a String:
a6 = do {xs <- evaluate ("text"++error "emsg"); print xs}
a7 = do {xs <- return ("text"++error "emsg"); print xs}
which both rise an exception. (In both cases the execution of print rises
it.) Which is different from
a8 = do {xs <- evaluate (error "emsg"); print xs}
where evaluate raises the exception.
a9 = do {xs <- return (error "emsg"); print xs}
has the same output as a8 (which is different than a6 and a7) but is raised
by print again.

This all is not of great interest if you do not try to catch the possible
exception. If you also want to get accurate strings it gets complicated.
If you just try to catch an exception while printing
a10 = do {xs <- return ("text"++error "emsg"); catch (print xs) (\e -> print
You get the string mixed with the error message.
The same goes when you use
a11 = do {xs <- evaluate ("text"++error "emsg"); catch (print xs) (\e ->
print e)}
If your string starts with the error then
a12 = do {xs <- evaluate ((error "emsg") ++ "123"); catch (print xs) (\e ->
print e)}
still raises an exception while
a13 = do {xs <- return ((error "emsg") ++ "123"); catch (print xs) (\e ->
print e)}
gives only the error message.
So I would say the return version is closer to what I want.
What I really want is that I force all the exceptions and catch them. In the
above excamples this would be something like
a14 = do {xs <- return ("Text" ++ error "emsg" ++ "123"); catch (print
(using xs rnf)) (\e -> print e)}
a15 = do {xs <- return (using ("Text" ++ error "emsg" ++ "123") rnf); catch
(print xs) (\e -> print e)}
And instead of "return (using a rnf)" I would like to write "evaluate a".

To sum it up:
- For my usage "evaluate a" is equal to "return a" (What are the cases where
"evaluate a" is different from "return $! a"? Stated in the library
documentation (Control.Exception).)
- I don't see where evaluate in its current form is better than "return a"
- I think "evaluate a" should mean "return (using a rnf)"

Do I miss something important here?
Does anybody use evaluate in a way that depends on it current definition?

Thanks for any comments,

----- Original Message -----
From: "Duncan Coutts" <duncan.coutts at worc.ox.ac.uk>
To: "Andreas Marth" <Andreas-Haskell at gmx.net>
Cc: <glasgow-haskell-users at haskell.org>
Sent: Tuesday, November 21, 2006 1:34 PM
Subject: Re: evaluate to lazy?

> On Tue, 2006-11-21 at 09:53 +0100, Andreas Marth wrote:
> > Hi!
> >
> > With the following code:
> >
> > module Guess where
> >
> > import Prelude hiding (catch)
> > import Control.Exception (evaluate, catch)
> >
> > guess :: [String] -> IO String
> > guess sl = do res <- catch (evaluate (concat $ sl ++ [error "some error
> > message", "blah blah blah"]))
> >                                     (\e -> return ("error#"++show e))
> >                     return res               -- only for demonstration
> > like this normally a call to sysalloc comes here
> >
> > what will
> > a) guess []
> > b) guess [""]
> > c) guess [" "]
> >
> > return?
> (glossing over the issue of return in IO)
> a,b) "error#some error message"
> c) ' ' : error "error#some error message"
> note that c) is not _|_.
> What about this one:
> d) fmap head (guess [" "])
> yep, it returns ' ', no error.
> > The idea was to return the string, catch every error if any occour,
> > it into a string and prefix it with "error#" and return this string
> > The reason to do this is to create a stable DLL with the error handling
> > non haskell land.
> > At the moment every exception raised crashes the whole system, which is
> > unacceptable.
> > Unfortunately the c) case still raises an exception.
> > I think at least the library description needs a hint that 'evaluate ("
> > "++error "some error message")' does not raise the error (which I find
> > strange!) and hence catch won't catch it.
> > Is this behaivior really desired or should we consider it a bug?
> The point is that:
> _|_  /=  1 : 2 : _|_
> so doing
> evaluate (1 : error "argh")
> doesn't throw any error since (1 : error "argh") is not an error value
> (though it does contain one).
> > PS: I circumvent this issue by using 'rnf' and 'using' from
> > Control.Parallel.Strategies
> That's a sensible strategy since then you force the whole list and will
> uncover any _|_ values inside the data structure.
> Duncan

More information about the Glasgow-haskell-users mailing list