[Haskell-cafe] Why is the no tuple syntax for sum types

[htebalaka]

FWIW there's already a somewhat extensible way to do this with singletons, as
shown here http://lpaste.net/137724. Could be made somewhat nicer with
pattern synonyms as well. It does at least solver the pattern matching issue
of nested Eithers.

Silvio Frischknecht wrote
>> For sum types you need a way to construct them and pattern match them.
>> Without a way to _name_ them, it would probably look like this:
>> 
>>     showError :: (Result | Error3 | Error8) -> String
>>     showError (result ||)   = "no error"
>>     showError (| error3 |)  = "error3: " ++ show error3
>>     showError (|| error8 |) = "error8: " ++ show error8
>> 
>> It's not the most readable code, so I'd generally avoid using it for
>> anything greater than 3 (same applies to tuples).
> 
> I doubt that functions usually cause more than one or two different
> errors. Unless you count undefined, asyncronous ... But you don't want
> those in your type anyway (too verbose).
> 
>> 
>> It also suffers the same flaw as tuples: no way to extend them easily.
>> Imagine adding a new error: you have to edit _all_ of the patterns:
>> 
>>     showError :: (Result | Error3 | Error8 | Error11 ) -> String
>>     showError (result |||)   = "no error"
>>     showError (| error3 ||)  = "error3: "  ++ show error3
>>     showError (|| error8 |)  = "error8: "  ++ show error8
>>     showError (||| error11 ) = "error11: " ++ show error11
>> 
>> Extensible records and variants might be able to solve these problems,
>> but that's a rabbit-hole of its own though.
>> 
> 
> Yes of course, but wouldn't you still agree that tuples are useful. So
> would sum tuples be.
> 
> Additionally, tuple like structures can easily be simulated using type
> operators
> 
> data a :' b = a :' b
> 
> However, the pattern matching for even a 3-way sum type using Either
> will be a nightmare. And I see no way of simulating this without using
> Template Haskell.
> 
> type ... a = a `Either` Int `Either` Char
> 
> Left (Right int) ???
> 
> or
> 
> Right (Left int) ???
> 
> Silvio
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