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 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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