Documentation

Extension for Variants prefers the first value

(l .=. Nothing) .*. v = v
(l .=. Just e)  .*. _ = mkVariant l e Proxy

Nothing .*. x = x
Just a .*. y = mkTIC a

>>> let labelX = Label :: Label "x"
>>> let labelY = Label :: Label "y"
>>> let p = labelX .*. labelY .*. emptyProxy
>>> :t p
p :: Proxy '["x", "y"]

Mixing two label kinds means we have to include Label:

>>> let s = label6 .*. label3 .*. emptyProxy
>>> :t s
s :: Proxy '[Label "6", Label (Lbl 'HZero () ())]

Mixing two label kinds means we have to include Label:

>>> let r = label3 .*. label6 .*. emptyProxy
>>> :t r
r :: Proxy '[Label (Lbl 'HZero () ()), Label "6"]

If possible, Label is left off:

>>> let q = label3 .*. label3 .*. emptyProxy
>>> :t q
q :: Proxy '[Lbl 'HZero () (), Lbl 'HZero () ()]

to keep types shorter, .*. used with Proxy avoids producing a Proxy :: Proxy '[Label x,Label y,Label z] if Proxy :: Proxy '[x,y,z] is not a kind error (as it is when mixing Label6 and Label3 labels).

ghc-7.6 does not accept Proxy ('[] :: [k]) so for now require k ~ *

if the proxy has Data.HList.Label3.Lbl, then everything has to be wrapped in Label to make the kinds match up.

Together with the instance below, this allows writing

makeLabelable "x y z"
p = x .*. y .*. z .*. emptyProxy

Or with HListPP

p = `x .*. `y .*. `z .*. emptyProxy

instead of

p = Proxy :: Proxy ["x","y","z"]

Subtyping for records

Subtyping for TIPs

(.*.)

Add a field to a record. Analagous to (++) for lists.

record .*. field1
       .*. field2

One occurrence and nothing is left

This variation provides an extra feature for singleton lists. That is, the result type is unified with the element in the list. Hence the explicit provision of a result type can be omitted.

Map a type to a natural (index within the collection) This is a purely type-level computation

And lift to the list of types

should this instead delete the first element of that type?

>>> let x :: Record '[Tagged "x" Int]; x = undefined
>>> let y :: Record '[Tagged "x" Char]; y = undefined
>>> :t hZip x y
hZip x y :: Record '[Tagged "x" (Int, Char)]

Missing from GHC-7.6.3 due to a bug:

let r = hEnd $ hBuild 1 2 3
*Data.HList> hZipList r r
H[(1,1),(2,2),(3,3)]
*Data.HList> hZip r r

<interactive>:30:1:
    Couldn't match type `Label k l' with `Integer'
    When using functional dependencies to combine
      HUnzip
        (Proxy [*]) ((':) * (Label k l) ls) ((':) * v vs) ((':) * lv lvs),
        arising from the dependency `xy -> x y'
        in the instance declaration at Data/HList/HListPrelude.hs:96:10
      HUnzip
        HList
        ((':) * Integer ((':) * Integer ((':) * Integer ('[] *))))
        ((':) * Integer ((':) * Integer ((':) * Integer ('[] *))))
        ((':)
           *
           (Integer, Integer)
           ((':) * (Integer, Integer) ((':) * (Integer, Integer) ('[] *)))),
        arising from a use of `hZip' at <interactive>:30:1-4
    In the expression: hZip r r
    In an equation for `it': it = hZip r r