Safe Haskell | None |
---|---|

Language | Haskell2010 |

The HList library

(C) 2004, Oleg Kiselyov, Ralf Laemmel, Keean Schupke

Zipping and unzipping for (conceptually) lists of pairs.

Provides two alternative implementations

- class (HZipR (MapFst z) (MapSnd z) ~ z) => HUnZip z where
- type family HZipR x y :: [*]
- type family Fst a
- type family Snd a
- hTranspose :: (HZip3 a b c, HFoldr HZipF (HList es) l (HList b), HReplicateFD n (HList ([] *)) es, HLengthEq1 HNat a n, HLengthEq2 HNat a n, SameLength' * * (HReplicateR * n ()) a) => HList ((:) * (HList a) l) -> HList c
- class HZip3 x y l | x y -> l, l -> x y where
- data HZipF = HZipF

# zip

## functional dependency

moved to Data.HList.HList to avoid an orphan instance

## type family

`hZip2`

can be written as a standalone function, with an appropriate
type family to calculate the result type. However, that does not seem to
be the case for `hUnzip2`

, so to re-use some type functions the two are
in the same class.

class (HZipR (MapFst z) (MapSnd z) ~ z) => HUnZip z where Source

HZipR in the superclass constraint doesn't hurt, but it doesn't seem to be necessary

type family HZipR x y :: [*] Source

calculates something like:

[a] -> [b] -> [(a,b)]

can be used to give another type for `hZip2`

hZip2 :: HList a -> HList b -> HList (HZipR a b)

## utility type functions

do they belong somewhere else?

# transpose

hTranspose :: (HZip3 a b c, HFoldr HZipF (HList es) l (HList b), HReplicateFD n (HList ([] *)) es, HLengthEq1 HNat a n, HLengthEq2 HNat a n, SameLength' * * (HReplicateR * n ()) a) => HList ((:) * (HList a) l) -> HList c Source

this transpose requires equal-length HLists inside a HList:

`>>>`

`import Data.HList.HListPrelude`

`>>>`

`let ex = (1 .*. 2 .*. HNil) .*. ('a' .*. 'b' .*. HNil) .*. ( 3 .*. 5 .*. HNil) .*. HNil`

The original list:

`>>>`

H[H[1,2],H['a','b'],H[3,5]]`ex`

And transposed:

`>>>`

H[H[1,'a',3],H[2,'b',5]]`hTranspose ex`