[Haskell-cafe] reifying based-on type of a newtype or data

[John Lato]

What about Data.Typeable.typeRepArgs ?

typeRepArgs :: TypeRep -> [TypeRep]

Prelude Data.Data> typeRepArgs (typeOf Foo)
[Int,Foo]

For any function type, the head of typeRepArgs should be the type of the
first parameter.  For non-function types, it looks like typeRepArgs returns
an empty list.

For anything more complicated, I suspect you'll need Data/Generic/Template
Haskell.

John L.


On Mon, Oct 28, 2013 at 7:15 PM, AntC <anthony_clayden at clear.net.nz> wrote:

> [reposted from Beginners, where it met stoney silence.]
>
> So I have (or rather the user of my package has):
>
> > {-# LANGUAGE  DeriveDataTypeable    #-}
> >
> >    newtype Foo = Foo Int    deriving (Read, Show, Typeable, Data, ...)
> >    someFoo     = Foo 7
> >
>
> Note:
> * the `newtype` could be `data` -- if that would help.
> * this is _not_ a parameterised type, but a 'baked in' `Int`.
> * the data constr is named same as the type -- if that would help.
>
> I can ask for `typeOf someFoo` and get `Foo` OK.
> I can ask for `typeOf Foo`  and get `Int -> Foo` OK.
>
> If I ask for `typeOf (typeOf someFoo)` I get `TypeRep`.
> `typeOf (show $ typeOf someFoo`) gets me `[Char]` (aka `String`)
>
> So far very logical, but not very helpful.
>
> What I want is to get the based-on type baked inside `someFoo`
> -- that is: `Int`
> (It would also be handy to get the name of the data constr, just in case
> it's different to the type.)
>
> Do I need to get into `deriving (..., Generic)` ?
>
> That looks like serious machinery!
>
> Thanks
> AntC
>
>
>
>
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