[Haskell-cafe] retrospective on 'seq' -> 'unsafeSeq' ?

pepe mnislaih at gmail.com
Mon Apr 14 07:22:42 EDT 2008

On 14/04/2008, at 12:19, Simon Peyton-Jones wrote:
> | > type constraints accordingly. (Analogously there could be an  
> unsafeShow that
> | > allows showing offending values in an 'error' without adding a  
> Show
> | > constraint to the type signature.)
> |
> | Ideally, unsafeShow could also show types as they are underneath,  
> not
> | as a pretty-printing Show might show them. I have often wanted to
> | overload Show to print things in a readable way, but to have a  
> showRaw
> | which shows things as they are, for debugging purposes. I have even
> | written such code for Yhc:
> | http://www.cs.york.ac.uk/fp/yhc/snapshot/docs/Yhc-Core-ShowRaw.html
> |
> | I think unsafeShow is a fantastic idea - and would be much more  
> useful
> | to me than unsafeSeq - plus is a non-breaking change. I think Hugs
> | already has 90% of the code to support this, and GHCi's debugger I
> | think has a fair chunk of it, so it could be added given not too  
> much
> | work.
> Yes, as you say, the debugger has most of the machinery.  I just  
> don't know what it'd take to make it a callable function. Pepe?
> Someone might want to make a feature-request ticket for this, with  
> as much background and/or suggested design as poss.

unsafeShow sounds quite useful, especially to avoid adding a Show  
constraint in function signatures only for debugging (of course a  
decent refactoring tool for Haskell would help with this too, so I  
hope the HaRe SoC project proposal gets accepted and done!).

:print has the code for doing this, but it needs the type information  
collected by the compiler. In GHC API speak, it needs the HscEnv from  
the Session object. If we can expose the Session created for GHCi  
(how? exporting it from GHC.Main? in a thread-local object? FFI  
trickery?), then this would need nearly zero work, albeit it would  
print things only when working in GHCi of course. But you can still  
compile all your modules to object code and call main from GHCi, so I  
don't think this is a big restriction considering unsafeShow is only  
for debugging purposes.

Another question is where in the package hierarchy would this function  
live. Since the code it would use is in the ghc package, it would  
introduce a dependency on it. And I am fairly sure that there is no  
package in the standard ghc distribution which depends on the ghc  
package. Ian, can it be made to live in ghc-prim without creating a  
dependency on the ghc package?

Alternatively, with some effort one can create a type-agnostic version  
of unsafeShow, which would print things in a more raw format, but  
probably sufficient anyway. I don't think it would work with unboxed  
values in general, although it can be made to work with the standard  
types. Actually, Bernie Pope wrote some code for this [1, see GHC Heap  
Printing library] some time ago, although with the new primitives and  
changes made for :print in GHC 6.8, this would be way easier nowadays.  
No need to use StablePtrs, no need to turn on profiling, and above  
all, no C needed :)
And as a bonus this would work out of GHCi too.

If there is a clean way to access the Session object, the first option  
means less implementation work, less code to maintain in GHC, and  
better functionality. What does the GHC team think?

[1] - http://www.cs.mu.oz.au/~bjpop/code.html

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