suggestion: add a .ehs file type

Simon Marlow simonmarhaskell at
Wed Nov 28 06:01:47 EST 2007

I see, all you're saying is you'd like the default to be different.

(That's not the same as saying "Extensions that change syntax are 
effectively declared by the use of that syntax", which is what you said 
earlier, BTW.)

Well, we could change the default.  I don't think it's a great idea 
personally - I think we should default to compiling whatever is the most 
recent standard, i.e. Haskell 98.  But you're arguing that the proportion 
of Haskell 98 code that would fail to compile is relatively small; that 
might well be true.  This isn't a decision we could take lightly, though.

Furthermore, it's only something we could change in 6.10, by which time it 
is likely that we'll have a clearer idea of what Haskell' is, so there 
might well be a -fhaskell-prime flag (or it might even be the default).


Alex Jacobson wrote:
> Simon, I think we've been trying to be too clever...
> The simple question is: for a given extension, what is the risk of 
> leaving it turned on by default?
> Clearly we don't want extensions turned on that causes code to compile 
> but with a different meaning.  We may not want extensions turned on that 
> cause most reasonable code not to compile.
> But I would say neither risk is significant in the case of most 
> extensions.  To use your examples:
> * FFI doesn't not cause h98 code to compile to a different meaning.  The 
> worst case is that code that uses 'foreign' as a function name doesn't 
> compile.  That seems okay in that more code probably uses FFI than uses 
> foreign as a function name and the user can apply a language pragma to 
> turn it off if really desired.
> * Existential types won't cause h98 code to compile with a different 
> meaning.  The worst case is that code that uses 'forall' as a type 
> variable won't compile.  That seems ok....
> * TemplateHaskell also not compatible with h98.  The worst case is the 
> loss "[d|" in list comprehensions.
> * MagicHash: Does not appear in the ToC or the Index of the user's guide 
> so should probably be turned off.  I have no idea what it does.
> Note, in all cases where the extension is turned on by default, there 
> should be a language pragma to turn it off.
> -Alex-
> Simon Marlow wrote:
>> Alex Jacobson wrote:
>>> Simon, from what I can tell, with GHC 6.8.1, use of foreign as a 
>>> function name or forall as a type variable or leaving out a space in 
>>> a list-comprehension doesn't "parse differently" when the relevant 
>>> extensions are enabled, it causes a parse error.
>>  >
>>> Extensions allow the same code to parse but with different meanings 
>>> need to be declared explicitly.  But, extensions that are obvious 
>>> from syntax should be allowed to be declared simply from the use of 
>>> that syntax.
>> So for the first example I gave,
>> f x y = x 3# y
>> the "MagicHash" extension is one that you'd require to be explicitly 
>> declared, because the expression parses both with and without the 
>> extension.
>> Now, Let's take the Template Haskell example:
>> f x = [d|d<-xs]
>> So this is valid Haskell 98, but invalid H98+TH.  You would therefore 
>> like this example to parse unambiguously as H98, correct?  But in 
>> order to do that, our parser would need arbitrary lookahead: it can't 
>> tell whether the expression is legal H98+TH until it gets to the '<-' 
>> in this case. Certainly it's possible to implement this using a 
>> backtracking parser, but Haskell is supposed to be parsable with a 
>> shift-reduce parser such as the one GHC uses.  Or we could try parsing 
>> the whole module with various combinations of extensions turned on or 
>> off, but I'm sure you can see the problems with that.
>> So basically the problem is that you need a parser that parses a 
>> strict superset of Haskell98 - and that's hard to achieve.
>> Cheers,
>>     Simon
>>> I am not taking a position here on the merits of any extensions.
>>> -Alex-
>>> Simon Marlow wrote:
>>>> Alex Jacobson wrote:
>>>>> Extensions that change syntax are effectively declared by the use 
>>>>> of that syntax.  If you can parse the source, then you know which 
>>>>> extensions it uses.
>>>> I thought we'd already established that this isn't possible.  Here 
>>>> are some code fragments that parse differently depending on which 
>>>> extensions are enabled:
>>>> f x y = x 3# y
>>>> f x = [d|d<-xs]
>>>> foreign x = x
>>>> f :: forall -> forall -> forall
>>>> You could argue that these syntax extensions are therefore badly 
>>>> designed, but that's a separate discussion.
>>>> Cheers,
>>>>     Simon
>>>>> -Alex-
>>>>> Duncan Coutts wrote:
>>>>>> On Fri, 2007-11-23 at 16:26 +0100, Wolfgang Jeltsch wrote:
>>>>>>> Am Freitag, 23. November 2007 03:37 schrieben Sie:
>>>>>>>> On Fri, 2007-11-23 at 01:50 +0100, Wolfgang Jeltsch wrote:
>>>>>>>>> Dont’t just think in terms of single modules.  If I have a 
>>>>>>>>> Cabal package,
>>>>>>>>> I can declare used extensions in the Cabal file.  A user can 
>>>>>>>>> decide not
>>>>>>>>> to start building at all if he/she sees that the package uses an
>>>>>>>>> extension unsupported by the compiler.
>>>>>>>> Indeed. In theory Cabal checks all the extensions declared to be 
>>>>>>>> used by
>>>>>>>> the package are supported by the selected compiler. In practise 
>>>>>>>> I'm not
>>>>>>>> sure how well it does this or what kind of error message we get.
>>>>>>> The problem is, of course, that you are not forced to specify all 
>>>>>>> used extensions in the Cabal file since you can still use 
>>>>>>> language pragmas.  Sometimes it is even desirable to use LANGUAGE 
>>>>>>> pragmas instead of information in the Cabal file.  For example, 
>>>>>>> even if some modules use undecidable instances, I might not want 
>>>>>>> all modules of the package to be compiled with 
>>>>>>> -XUndecidableInstances since this could hide problems with my 
>>>>>>> class structure.
>>>>>> Our tentative plan there is to separate the extensions field into 
>>>>>> those
>>>>>> used in some module, and those applied by cabal to every module. 
>>>>>> So that
>>>>>> would allow you to specify a feature in one file but not all, while
>>>>>> still declaring to the outside world that the package uses the 
>>>>>> feature.
>>>>>> As for enforcing that, that may come almost for free when we get
>>>>>> dependency chasing as we'll be looking for imports anyway. It 
>>>>>> shouldn't
>>>>>> be much harder to look for language pragmas too.
>>>>>> Duncan
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