[Haskell-cafe] some questions about Template Haskell

[Richard Eisenberg]

Hi TP,

The reason that your initial example doesn't work is that Template Haskell splices can be used in four places: expressions, types, patterns (I think), and top-level declarations. The number in a fixity declaration is none of these. It's not an expression because you must write a literal number. However, the fixity declaration itself can be produced by a splice, and you've discovered that way out.

About your first issue, I don't quite know what's going on there, either, I'm afraid.

Richard

On Jun 29, 2013, at 9:03 PM, TP wrote:

> TP wrote:
> 
>> 2/ If I define in a module:
>> 
>> j = 3
>> 
>> and then define in another module:
>> 
>> -------------------
>> h x = $([|j|])
>> main = do
>> print $ h undefined
>> -------------------
>> 
>> I obtain "3" as expected.
>> 
>> However, I do not achieve to make this system work with an infix
>> declaration:
>> 
>> infix $([| j |]) +
>> 
>> I obtain:
>> 
>> parse error on input `$('
> 
> I don't know what happens exactly, but one way to get out of this problem is 
> to write the complete top-level declaration with a splice, instead of only 
> the fixity level:
> 
> $(return $ [ InfixD (Fixity $([| j |]) InfixN) (mkName "+") ])
> 
> Concerning my first question, I have not been able to understand what 
> happens at this time. I continue to look at it.
> 
> Thanks,
> 
> TP
> 
> 
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