[Hs-Generics] FW: [Haskell-cafe] SYB vs HList (again)

[S. Alexander Jacobson]

Oleg,

>> data Name      = Name String String deriving Show
>> newtype Salary = S Float deriving Show
>> data Dept      = D String Int deriving Show

I like the idea of these simpler label declarations, but I think the 
cost is that it is harder to write generic code for e.g. parsing 
urlencoded data or generating XML.  If I understand your concept here 
correctly, you need to write not only the label declaration but also 
dedicated code for parsing/generating with each label type.  For 
example, how do you extract a Name above from a urlencoded string?

Yes the hlist label declaraions I am using are very verbose, but they 
are easy to tighten with template haskell (even if I think template 
haskell is ugly).

>> It would be really nice if there was some way to tell Haskell that
>> HLists have no more fields than the ones you happen to be getting and
>> setting in your code. Effectively that would mean you get data
>> structure inference not just function type inference which would be
>> really cool!
>
> I'm not sure I follow. Could you outline an example of the code you
> wish work? Incidentally, a lot of the library depends on the record
> types being members of some specific classes. One can define
>

I'd like be able to do something like this:

   $(label Salary Int) -- template haskell to define salary label
   main = do
          person <- readFile "blah" >>= return . read
          print $ person # salary


In this case, haskell would assume that person has only one label, 
salary.  The read function would ignore all the other labels.   If I 
changed the code to this:

   $(label Salary Int) -- template haskell to define salary label
   $(label Name String)

   main = do
          person <- readFile "blah" >>= return . read
          print $ person # salary
          print $ show (person::Name .*. Salary)

Then the code would assume that a person has both a name and a salary.

Conceptually, I think what I really want is the data structure 
equivalent of type inference.  Just as I don't want to be forced to 
declare my function types, I don't want to be forced to declare my 
data types.  The field labels I use should be enough to define the 
shape of my type.  The reason this is really important is that if 
hlists contain hlists, the type declarations can get really really 
messy

Separately, I would really like hrecords not to have order dependency. 
It seems strange to me that (Foo .*. Bar .*. HNil) is a different type 
from (Bar .*. Foo .*. HNil).

If there is no way to get the type system to do this, I think I will 
probably use template haskell to declare the types of HList records. 
I assume I can make template haskell always sort the labels before 
generating the type code.

-Alex-





On Thu, 15 Mar 2007, oleg at pobox.com wrote:

>
> [Please follow-up to generics at haskell.org]
>
> S. Alexander Jacobson wrote:
>> HLists require you to define Labels and basically only use label
>> values that are themselves either scalar or HLists.
>> ...
>> With SYB you create field labels using newtype (or data) declarations
>> e.g.
>>
>>    data Salary = S {salary::Float}
>>
>> With HList, label declarations are really verbose e.g.
>>
>>    data SalaryLabel deriving(Typeable)
>>    type Salary = Field (Proxy SalaryLabel) Int
>>    salary = proxy :: Proxy FooLabel
>
> Actually there is no requirement that HList record names must be
> scalar `labels', must be Proxies and require such a complex
> declaration. From HList's high point of view, any collection can be a
> record provided the type of each item is unique and there is some way
> to extract the value associated with that type. The HList library
> provides two implementations of Records (and there was one more,
> obsolete now). There could be more. For example, I have just committed
> a yet another implementation,
> 	http://darcs.haskell.org/HList/src/RecordD.hs
> Here a record is a list of things that have a type and a value and
> provide a way to extract that value. The example from the end of this
> file seems worth quoting:
>
>> data Name      = Name String String deriving Show
>> newtype Salary = S Float deriving Show
>> data Dept      = D String Int deriving Show
>>
>> person = (Name "Joe" "Doe") .*. (S 1000) .*. (D "CIO" 123) .*. emptyRecord
>>
>> -- could be derived automatically, like Typeable...
>> instance Fieldish Name (String,String) where
>>     fromField (Name s1 s2) = (s1,s2)
>> instance Fieldish Salary Float where
>>     fromField (S n) = n
>> instance Fieldish Dept (String,Int) where
>>     fromField (D s n) = (s,n)
>>
>> test1 = show person
>> -- When a field acts as a label, only its type matters, not the contents
>> test2 = person .!. (Name undefined undefined)
>> test3 = person .!. (undefined::Salary)
>> test5 = person .!. (D "xxx" 111)
>
>
>
>> I don't know exactly how HList handles default values but I assume you
>> can restrict use of those values to explicit deserialization contexts.
>> Is that correct?
>
> I'm not sure what you mean about the restriction of default values to
> deserialization contexts. Anyway, HList provides a left-biased union
> of two records: hLeftUnion r1 r2 is the record r1 augmented with all
> the fields from r2 that didn't occur in r2. One may consider r2 to be
> the record with default fields and the corresponding values.
>
>> It would be really nice if there was some way to tell Haskell that
>> HLists have no more fields than the ones you happen to be getting and
>> setting in your code. Effectively that would mean you get data
>> structure inference not just function type inference which would be
>> really cool!
>
> I'm not sure I follow. Could you outline an example of the code you
> wish work? Incidentally, a lot of the library depends on the record
> types being members of some specific classes. One can define
>
>> newtype ClosedRecord = ClosedRecord r
>
> To make a ClosedRecord to be a record from which we can extract the
> values of some fields, we merely need to say
>> instance HasField l r v => HasField l (ClosedRecord r) v
>> where hLookupByLabel l (ClosedRecord r) v = hLookupByLabel l r v
>
> Since we did not make this record the member of HExtend or HAppend, it
> is not extensible.
>
>