[Haskell] modern language design, stone age tools

Simon Peyton-Jones simonpj at microsoft.com
Thu Jun 24 04:03:54 EDT 2004

Switching on profiling (-prof -auto-all) does essentially just this. The
cost centre stack is just like an implicit parameters, only one that
does not show up in the types.

Since there really is some extra parameter passing going on, you really
do need to recompile (all) the code.  That's a nuisance, but it's a
price that I can't see how to avoid in a call-by-need language.


| -----Original Message-----
| From: haskell-bounces at haskell.org [mailto:haskell-bounces at haskell.org]
On Behalf Of Alastair Reid
| Sent: 23 June 2004 21:44
| To: S. Alexander Jacobson
| Cc: haskell at haskell.org
| Subject: Re: [Haskell] modern language design, stone age tools
| On Wednesday 23 June 2004 20:38, S. Alexander Jacobson wrote:
| > It would be really nice if you could pass an
| > error message down to every function that might
| > fail. e.g. using implicit parameters*:
| >
| >    myFunc 0 x = head x with ?failMsg="myfunc 0 caused the error"
| Interesting.  Two variations on that:
| 1) Getting a 'stack' trace (i.e., a list of ancestors) can be more
|    useful than just knowing the immediate ancestor so you might want
|    myFunc to also take an implicit parameter and to add that
|    parameter to what it says:
|      myFunc 0 x = head x
|       with ?failMsg="myfunc 0 caused the error but myFunc was called
because "
|                    ++ ?failMsg
|    [I can't remember if with acts as a let or a letrec.  The intention
|     to use the ?failMsg passed implicitly to myFunc]
| 2) If you don't want to put errors in the type system, you could
instead use
|    exceptions something along the lines of:
|       myFunc 0 x = mapException
|                      (\ err -> show err ++ "when invoked by myFunc 0")
|                      (head x)
|    [Not quite type correct but hopefully clear enough.  The idea is to
|     the exception value returned with some extra information about the
|     with the idea that whoever called myFunc might add extra
|     Ideally, the Exception type would be recursive so we could build
chains of
|     exceptions without having to use Show.]
| --
| Alastair Reid
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