Adding type signature changes semantics (was [Haskell-cafe] Lazy in either argument?)

[Stefan O'Rear]

On Fri, Aug 03, 2007 at 09:01:18PM -0300, Isaac Dupree wrote:
> Tim Chevalier wrote:
>> On 8/3/07, Simon Peyton-Jones <simonpj at microsoft.com> wrote:
>>> Stefan is right here.
>>>
>>> - It's not surprising that with -Onot you get different code from 
>>> different source programs, even if one can readily be transformed into 
>>> the other.  That's what -O does.
>>>
>> Yes, but I found it surprising that just removing a type signature
>> should result in markedly different code. Are there other known
>> situations where that can happen?
>
> It is not _just_ removing a type signature, it is also changing the type 
> from `Bool` to `forall a. a`. An explicit type signature of the latter 
> would have produced the same results as no type signature, I believe. The 
> surprise is that an unconstrained type-variable being variable rather than 
> instantiated to an arbitrary type, makes any difference (since it doesn't, 
> normally, at runtime).  I would guess the programs `Bool` and `a` are the 
> same once optimizations are turned on?  Maybe GHC could avoid the creation 
> of type-lambdas that are unused (in some sense)... with -Onot... I'm 
> dubious about that.

It *does not change the designed semantics at all*.

It *tickles a bug*.

In the *absence of code generator bugs* it generates *semantically
equivalent code*.

Do you want inserting or removing type signatures to always tickle the
same bugs?  Seems like an awfully constraining desgin choice to me.

> Inserting a preemption test in non-allocating loops seems like a good idea 
> to me (I hate the invisible threat that my program might not thread as 
> threading should work)... any idea how bad the performance impact could be 
> (I guess the test could be specified to branch-predict that the loop 
> wouldn't be interrupted), and whether there could be a pragma to disable 
> that test in certain loops? Is -threaded versus not, relevant here?
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