Data.Generics.Fixplate.Hash

Generic hashing on trees. We recursively compute hashes of all subtrees, giving fast inequality testing, and a fast, but meaningless (more-or-less random) ordering on the set of trees (so that we can put them into Map-s).

The way it works is that when we compute the hash of a node, we use the hashes of the children directly; this way, you can also incrementally build up a hashed tree.

Hashed tree type

data HashAnn hash f a

Hash annotation (question: should the Hash field be strict? everything else in the library is lazy...)

This is custom datatype instead of reusing Ann because of the different Eq/Ord instances we need.

Constructors

HashAnn hash (f a)

Instances

Functor f => Functor (HashAnn hash f)

Foldable f => Foldable (HashAnn hash f)

Traversable f => Traversable (HashAnn hash f)

(ShowF f, Show hash) => ShowF (HashAnn hash f)

(Ord hash, OrdF f) => OrdF (HashAnn hash f)

(Eq hash, EqF f) => EqF (HashAnn hash f)

(Show hash, Show (f a)) => Show (HashAnn hash f a)

getHash :: HashAnn hash f a -> hash

unHashAnn :: HashAnn hash f a -> f a

type HashMu hash f = Mu (HashAnn hash f)

A tree annotated with hashes of all subtrees. This gives us fast inequality testing, and fast (but meaningless!) ordering for Map-s.

topHash :: HashMu hash f -> hash

The hash of the complete tree.

forgetHash :: Functor f => HashMu hash f -> Mu f

Interface to the user's hash functions

data HashValue hash

A concrete hash implementation. We don't use type classes since

• a hash type class does not belong to this library;
• we don't want to restrict the user's design space

Thus we simulate type classes with record types.

Constructors

HashValue

Fields

_emptyHash :: hash

the hash of an empty byte sequence

_hashChar :: Char -> hash -> hash

digest a (unicode) character

_hashHash :: hash -> hash -> hash

digest a hash value

Hashing tres

hashTree :: (Foldable f, Functor f, ShowF f) => HashValue hash -> Mu f -> HashMu hash f

This function uses the ShowF instance to compute the hash of a node; this way you always have a working version without writing any additional code.

However, you can also supply your own hash implementation (which can be more efficient, for example), if you use hashTreeWith instead.

hashTreeWith :: (Foldable f, Functor f) => HashValue hash -> (f Hole -> hash -> hash) -> Mu f -> HashMu hash f

hashNode :: (Foldable f, Functor f, ShowF f) => HashValue hash -> f (HashMu hash f) -> HashMu hash f

Build a hashed node from the children.

hashNodeWith :: (Foldable f, Functor f) => HashValue hash -> (f Hole -> hash -> hash) -> f (HashMu hash f) -> HashMu hash f