Documentation

In March 2010, Andrew Frank extended the problem for monadic operations. This is the monadic version of TIPTransform.hs in the present directory.

This is the TF implementation. When specifying the operation to perform over a TIP, we can leave it polymorphic over the monad. The type checker will instantiate the monad based on the context.

Transforming a TIP: applying to a TIP a (polyvariadic) function that takes arguments from a TIP and updates the TIP with the result.

In more detail: we have a typed-indexed collection TIP and we would like to apply a transformation function to it, whose argument types and the result type are all in the TIP. The function should locate its arguments based on their types, and update the TIP with the result. The function may have any number of arguments, including zero; the order of arguments should not matter.

The problem was posed by Andrew U. Frank on Haskell-Cafe, Sep 10, 2009. http://www.haskell.org/pipermail/haskell-cafe/2009-September/066217.html The problem is an interesting variation of the keyword argument problem.

Examples can be found in examples/TIPTransform.hs and examples/TIPTransformM.hs

Sometimes the type variables available have TagR already applied (ie the lists have elements like Tagged X X). Then this abbreviation is useful:

TagR can also be used to avoid redundancy when defining types for TIC and TIP.

 type XShort = TagR [A,B,C,D]

 type XLong = [Tagged A A, Tagged B B, Tagged C C, Tagged D D]

an equivalent FD version, which is slightly better with respect to simplifying types containing type variables (in ghc-7.8 and 7.6): http://stackoverflow.com/questions/24110410/

With ghc-7.10 (http://ghc.haskell.org/trac/ghc/ticket/10009) the FD version is superior to the TF version:

class (UntagR (TagR a) ~ a) => TagUntag a where
    type TagR a :: [*]
    hTagSelf :: HList a -> HList (TagR a)
    hUntagSelf :: HList (TagR a) -> HList a

instance TagUntag '[] where
    type TagR '[] = '[]
    hTagSelf _ = HNil
    hUntagSelf _ = HNil

instance TagUntag xs => TagUntag (x ': xs) where
    type TagR (x ': xs) = Tagged x x ': TagR xs
    hTagSelf (HCons x xs) = Tagged x HCons hTagSelf xs
    hUntagSelf (HCons (Tagged x) xs) = x HCons hUntagSelf xs

type family UntagR (xs :: [*]) :: [*]
type instance UntagR '[] = '[]
type instance UntagR (x ': xs) = Untag1 x ': UntagR xs

Length information should flow backwards

>>> let len2 x = x `asTypeOf` (undefined :: HList '[a,b])
>>> let f = len2 $ hTagSelf (hReplicate Proxy ())
>>> :t f
f :: HList '[Tagged () (), Tagged () ()]

this constraint ensures that a TIP created by mkTIP has no duplicates

TIPs are like Record, except element "i" of the list "l" has type Tagged e_i e_i

Use Labels to specify the first argument

provides a Lens' (TIP s) a. hLens' :: Label a -> Lens' (TIP s) a is another option.

provides a Lens (TIP s) (TIP t) a b

When using set (also known as .~), tipyLens' can address the ambiguity as to which field "a" should actually be updated.

The same as tipyProject, except also return the types not requested in the proxy argument

Iso (TIP (TagR a)) (TIP (TagR b)) (HList a) (HList b)

Iso' (TIP (TagR s)) (HList a)

Iso (TIP s) (TIP t) (Record s) (Record t)

typeIndexed may be more appropriate

Iso' (TIP (TagR s)) (Record a)

specialization of hZip

specialization of hUnzip