[Haskell-cafe] What's the new type ?

[zaxis]

very clear !  thank you


edwardk wrote:
> 
> Given a newtype declaration:
> 
> newtype Foo a = Bar (...)
> 
> The newtype is Foo a, and it uses a constructor Bar, which gets 'erased'
> at
> compile time, unlike a data declaration. By convention, usually Foo and
> Bar
> are the same thing. In this case the constructor for GenParser is named
> Parser instead.
> 
> To understand the GenParser type, you must consider that originally,
> 'GenParser' probably didn't exist. And if it did, there is a pedagogical
> justification to just explaining the simpler 'Parser' case first without
> appealing to the notion of a parser in its full generality.
> 
> So if you started from a type like
> 
> newtype Parser a = Parser (State Char () -> Consumed (Reply Char () a))
> 
> and later want to generalize that to a more permissive signature, without
> breaking all of the code that uses that constructor, then the upgrade path
> for that code is to keep the same constructor name, but generalize the
> type.
> 
> So Parser becomes a type alias:
> 
> type Parser = GenParser Char ()
> 
> and GenParser is introduced as a newtype, which happens to use the
> constructor Parser for the dual reasons of backwards compatibility and so
> that people working on simple parsers don't need to think about
> alternative
> user state and token types.
> 
> newtype GenParser tok st a = Parser (State tok st -> Consumed (Reply tok
> st
> a))
> 
> Now the only thing that breaks is that any code that previously defined
> instances for Parser before the notion of GenParser must add a LANGUAGE
> pragma indicating that TypeSynonymInstances are allowed.
> 
> -Edward Kmett
> 
> On Thu, Nov 5, 2009 at 2:17 AM, zaxis <z_axis at 163.com> wrote:
> 
>>
>> type Parser a = GenParser Char () a
>> newtype GenParser tok st a = Parser (State tok st -> Consumed (Reply tok
>> st
>> a))
>>
>> As i know, the Parser type is just an alias of GenParser. Then  can the
>> second line be replaced as below?
>>
>> newtype GenParser tok st a = GenParser Char () (State tok st -> Consumed
>> (Reply tok st a))
>>
>> If it can , then what is the new type ?
>>
>> Sincerely!
>>
>> -----
>> fac n = foldr (*) 1 [1..n]
>> --
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>>
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-----
fac n = foldr (*) 1 [1..n]
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Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.