path: root/stdlib/source/lux/data/collection/dictionary.lux

(.module:
  [lux #*
   [abstract
    [hash (#+ Hash)]
    [equivalence (#+ Equivalence)]
    [functor (#+ Functor)]]
   [control
    ["." try (#+ Try)]
    ["." exception (#+ exception:)]]
   [data
    ["." maybe]
    ["." product]
    ["." number
     ["." i64]
     ["n" nat]]
    [collection
     ["." list ("#@." fold functor monoid)]
     ["." array (#+ Array) ("#@." functor fold)]]]])

## This implementation of Hash Array Mapped Trie (HAMT) is based on
## Clojure's PersistentHashMap implementation.
## That one is further based on Phil Bagwell's Hash Array Mapped Trie.

## Bitmaps are used to figure out which branches on a #Base node are
## populated. The number of bits that are 1s in a bitmap signal the
## size of the #Base node.
(type: BitMap
  Nat)

## Represents the position of a node in a BitMap.
## It's meant to be a single bit set on a 32-bit word.
## The position of the bit reflects whether an entry in an analogous
## position exists within a #Base, as reflected in it's BitMap.
(type: BitPosition
  Nat)

## An index into an array.
(type: Index
  Nat)

## A hash-code derived from a key during tree-traversal.
(type: Hash-Code
  Nat)

## Represents the nesting level of a leaf or node, when looking-it-up
## while exploring the tree.
## Changes in levels are done by right-shifting the hashes of keys by
## the appropriate multiple of the branching-exponent.
## A shift of 0 means root level.
## A shift of (* branching-exponent 1) means level 2.
## A shift of (* branching-exponent N) means level N+1.
(type: Level
  Nat)

## Nodes for the tree data-structure that organizes the data inside
## Dictionaries.
(type: (Node k v)
  (#Hierarchy Nat (Array (Node k v)))
  (#Base BitMap
         (Array (Either (Node k v)
                        [k v])))
  (#Collisions Hash-Code (Array [k v])))

## #Hierarchy nodes are meant to point down only to lower-level nodes.
(type: (Hierarchy k v)
  [Nat (Array (Node k v))])

## #Base nodes may point down to other nodes, but also to leaves,
## which are KV-pairs.
(type: (Base k v)
  (Array (Either (Node k v)
                 [k v])))

## #Collisions are collections of KV-pairs for which the key is
## different on each case, but their hashes are all the same (thus
## causing a collision).
(type: (Collisions k v)
  (Array [k v]))

## That bitmap for an empty #Base is 0.
## Which is the same as 0000 0000 0000 0000 0000 0000 0000 0000.
## Or 0x00000000.
## Which is 32 zeroes, since the branching factor is 32.
(def: clean-bitmap
  BitMap
  0)

## Bitmap position (while looking inside #Base nodes) is determined by
## getting 5 bits from a hash of the key being looked up and using
## them as an index into the array inside #Base.
## Since the data-structure can have multiple levels (and the hash has
## more than 5 bits), the binary-representation of the hash is shifted
## by 5 positions on each step (2^5 = 32, which is the branching
## factor).
## The initial shifting level, though, is 0 (which corresponds to the
## shift in the shallowest node on the tree, which is the root node).
(def: root-level
  Level
  0)

## The exponent to which 2 must be elevated, to reach the branching
## factor of the data-structure.
(def: branching-exponent
  Nat
  5)

## The threshold on which #Hierarchy nodes are demoted to #Base nodes,
## which is 1/4 of the branching factor (or a left-shift 2).
(def: demotion-threshold
  Nat
  (i64.left-shift (n.- 2 branching-exponent) 1))

## The threshold on which #Base nodes are promoted to #Hierarchy nodes,
## which is 1/2 of the branching factor (or a left-shift 1).
(def: promotion-threshold
  Nat
  (i64.left-shift (n.- 1 branching-exponent) 1))

## The size of hierarchy-nodes, which is 2^(branching-exponent).
(def: hierarchy-nodes-size
  Nat
  (i64.left-shift branching-exponent 1))

## The cannonical empty node, which is just an empty #Base node.
(def: empty
  Node
  (#Base clean-bitmap (array.new 0)))

## Expands a copy of the array, to have 1 extra slot, which is used
## for storing the value.
(def: (insert! idx value old-array)
  (All [a] (-> Index a (Array a) (Array a)))
  (let [old-size (array.size old-array)]
    (|> (array.new (inc old-size))
        (array.copy! idx 0 old-array 0)
        (array.write! idx value)
        (array.copy! (n.- idx old-size) idx old-array (inc idx)))))

## Creates a copy of an array with an index set to a particular value.
(def: (update! idx value array)
  (All [a] (-> Index a (Array a) (Array a)))
  (|> array array.clone (array.write! idx value)))

## Creates a clone of the array, with an empty position at index.
(def: (vacant! idx array)
  (All [a] (-> Index (Array a) (Array a)))
  (|> array array.clone (array.delete! idx)))

## Shrinks a copy of the array by removing the space at index.
(def: (remove! idx array)
  (All [a] (-> Index (Array a) (Array a)))
  (let [new-size (dec (array.size array))]
    (|> (array.new new-size)
        (array.copy! idx 0 array 0)
        (array.copy! (n.- idx new-size) (inc idx) array idx))))

## Increases the level-shift by the branching-exponent, to explore
## levels further down the tree.
(def: level-up
  (-> Level Level)
  (n.+ branching-exponent))

(def: hierarchy-mask BitMap (dec hierarchy-nodes-size))

## Gets the branching-factor sized section of the hash corresponding
## to a particular level, and uses that as an index into the array.
(def: (level-index level hash)
  (-> Level Hash-Code Index)
  (i64.and hierarchy-mask
           (i64.logic-right-shift level hash)))

## A mechanism to go from indices to bit-positions.
(def: (->bit-position index)
  (-> Index BitPosition)
  (i64.left-shift index 1))

## The bit-position within a base that a given hash-code would have.
(def: (bit-position level hash)
  (-> Level Hash-Code BitPosition)
  (->bit-position (level-index level hash)))

(def: (bit-position-is-set? bit bitmap)
  (-> BitPosition BitMap Bit)
  (not (n.= clean-bitmap (i64.and bit bitmap))))

## Figures out whether a bitmap only contains a single bit-position.
(def: only-bit-position?
  (-> BitPosition BitMap Bit)
  n.=)

(def: (set-bit-position bit bitmap)
  (-> BitPosition BitMap BitMap)
  (i64.or bit bitmap))

(def: unset-bit-position
  (-> BitPosition BitMap BitMap)
  i64.xor)

## Figures out the size of a bitmap-indexed array by counting all the
## 1s within the bitmap.
(def: bitmap-size
  (-> BitMap Nat)
  i64.count)

## A mask that, for a given bit position, only allows all the 1s prior
## to it, which would indicate the bitmap-size (and, thus, index)
## associated with it.
(def: bit-position-mask
  (-> BitPosition BitMap)
  dec)

## The index on the base array, based on it's bit-position.
(def: (base-index bit-position bitmap)
  (-> BitPosition BitMap Index)
  (bitmap-size (i64.and (bit-position-mask bit-position)
                        bitmap)))

## Produces the index of a KV-pair within a #Collisions node.
(def: (collision-index Hash<k> key colls)
  (All [k v] (-> (Hash k) k (Collisions k v) (Maybe Index)))
  (:: maybe.monad map product.left
      (array.find+ (function (_ idx [key' val'])
                     (:: Hash<k> = key key'))
                   colls)))

## When #Hierarchy nodes grow too small, they're demoted to #Base
## nodes to save space.
(def: (demote-hierarchy except-idx [h-size h-array])
  (All [k v] (-> Index (Hierarchy k v) [BitMap (Base k v)]))
  (product.right (list@fold (function (_ idx [insertion-idx node])
                              (let [[bitmap base] node]
                                (case (array.read idx h-array)
                                  #.None            [insertion-idx node]
                                  (#.Some sub-node) (if (n.= except-idx idx)
                                                      [insertion-idx node]
                                                      [(inc insertion-idx)
                                                       [(set-bit-position (->bit-position idx) bitmap)
                                                        (array.write! insertion-idx (#.Left sub-node) base)]])
                                  )))
                            [0 [clean-bitmap
                                (array.new (dec h-size))]]
                            (list.indices (array.size h-array)))))

## When #Base nodes grow too large, they're promoted to #Hierarchy to
## add some depth to the tree and help keep it's balance.
(def: hierarchy-indices (List Index) (list.indices hierarchy-nodes-size))

(def: (promote-base put' Hash<k> level bitmap base)
  (All [k v]
    (-> (-> Level Hash-Code k v (Hash k) (Node k v) (Node k v))
        (Hash k) Level
        BitMap (Base k v)
        (Array (Node k v))))
  (product.right (list@fold (function (_ hierarchy-idx (^@ default [base-idx h-array]))
                              (if (bit-position-is-set? (->bit-position hierarchy-idx)
                                                        bitmap)
                                [(inc base-idx)
                                 (case (array.read base-idx base)
                                   (#.Some (#.Left sub-node))
                                   (array.write! hierarchy-idx sub-node h-array)

                                   (#.Some (#.Right [key' val']))
                                   (array.write! hierarchy-idx
                                                 (put' (level-up level) (:: Hash<k> hash key') key' val' Hash<k> empty)
                                                 h-array)

                                   #.None
                                   (undefined))]
                                default))
                            [0
                             (array.new hierarchy-nodes-size)]
                            hierarchy-indices)))

## All empty nodes look the same (a #Base node with clean bitmap is
## used).
## So, this test is introduced to detect them.
(def: (empty?' node)
  (All [k v] (-> (Node k v) Bit))
  (`` (case node
        (#Base (~~ (static ..clean-bitmap)) _)
        #1

        _
        #0)))

(def: (put' level hash key val Hash<k> node)
  (All [k v] (-> Level Hash-Code k v (Hash k) (Node k v) (Node k v)))
  (case node
    ## For #Hierarchy nodes, I check whether I can add the element to
    ## a sub-node. If impossible, I introduced a new singleton sub-node.
    (#Hierarchy _size hierarchy)
    (let [idx (level-index level hash)
          [_size' sub-node] (case (array.read idx hierarchy)
                              (#.Some sub-node)
                              [_size sub-node]

                              _
                              [(inc _size) empty])]
      (#Hierarchy _size'
                  (update! idx (put' (level-up level) hash key val Hash<k> sub-node)
                           hierarchy)))

    ## For #Base nodes, I check if the corresponding BitPosition has
    ## already been used.
    (#Base bitmap base)
    (let [bit (bit-position level hash)]
      (if (bit-position-is-set? bit bitmap)
        ## If so...
        (let [idx (base-index bit bitmap)]
          (case (array.read idx base)
            #.None
            (undefined)

            ## If it's being used by a node, I add the KV to it.
            (#.Some (#.Left sub-node))
            (let [sub-node' (put' (level-up level) hash key val Hash<k> sub-node)]
              (#Base bitmap (update! idx (#.Left sub-node') base)))

            ## Otherwise, if it's being used by a KV, I compare the keys.
            (#.Some (#.Right key' val'))
            (if (:: Hash<k> = key key')
              ## If the same key is found, I replace the value.
              (#Base bitmap (update! idx (#.Right key val) base))
              ## Otherwise, I compare the hashes of the keys.
              (#Base bitmap (update! idx
                                     (#.Left (let [hash' (:: Hash<k> hash key')]
                                               (if (n.= hash hash')
                                                 ## If the hashes are
                                                 ## the same, a new
                                                 ## #Collisions node
                                                 ## is added.
                                                 (#Collisions hash (|> (array.new 2)
                                                                       (array.write! 0 [key' val'])
                                                                       (array.write! 1 [key val])))
                                                 ## Otherwise, I can
                                                 ## just keep using
                                                 ## #Base nodes, so I
                                                 ## add both KV-pairs
                                                 ## to the empty one.
                                                 (let [next-level (level-up level)]
                                                   (|> empty
                                                       (put' next-level hash' key' val' Hash<k>)
                                                       (put' next-level hash  key  val Hash<k>))))))
                                     base)))))
        ## However, if the BitPosition has not been used yet, I check
        ## whether this #Base node is ready for a promotion.
        (let [base-count (bitmap-size bitmap)]
          (if (n.>= ..promotion-threshold base-count)
            ## If so, I promote it to a #Hierarchy node, and add the new
            ## KV-pair as a singleton node to it.
            (#Hierarchy (inc base-count)
                        (|> (promote-base put' Hash<k> level bitmap base)
                            (array.write! (level-index level hash)
                                          (put' (level-up level) hash key val Hash<k> empty))))
            ## Otherwise, I just resize the #Base node to accommodate the
            ## new KV-pair.
            (#Base (set-bit-position bit bitmap)
                   (insert! (base-index bit bitmap) (#.Right [key val]) base))))))
    
    ## For #Collisions nodes, I compare the hashes.
    (#Collisions _hash _colls)
    (if (n.= hash _hash)
      ## If they're equal, that means the new KV contributes to the
      ## collisions.
      (case (collision-index Hash<k> key _colls)
        ## If the key was already present in the collisions-list, it's
        ## value gets updated.
        (#.Some coll-idx)
        (#Collisions _hash (update! coll-idx [key val] _colls))

        ## Otherwise, the KV-pair is added to the collisions-list.
        #.None
        (#Collisions _hash (insert! (array.size _colls) [key val] _colls)))
      ## If the hashes are not equal, I create a new #Base node that
      ## contains the old #Collisions node, plus the new KV-pair.
      (|> (#Base (bit-position level _hash)
                 (|> (array.new 1)
                     (array.write! 0 (#.Left node))))
          (put' level hash key val Hash<k>)))
    ))

(def: (remove' level hash key Hash<k> node)
  (All [k v] (-> Level Hash-Code k (Hash k) (Node k v) (Node k v)))
  (case node
    ## For #Hierarchy nodes, find out if there's a valid sub-node for
    ## the Hash-Code.
    (#Hierarchy h-size h-array)
    (let [idx (level-index level hash)]
      (case (array.read idx h-array)
        ## If not, there's nothing to remove.
        #.None
        node

        ## But if there is, try to remove the key from the sub-node.
        (#.Some sub-node)
        (let [sub-node' (remove' (level-up level) hash key Hash<k> sub-node)]
          ## Then check if a removal was actually done.
          (if (is? sub-node sub-node')
            ## If not, then there's nothing to change here either.
            node
            ## But if the sub-removal yielded an empty sub-node...
            (if (empty?' sub-node')
              ## Check if it's due time for a demotion.
              (if (n.<= demotion-threshold h-size)
                ## If so, perform it.
                (#Base (demote-hierarchy idx [h-size h-array]))
                ## Otherwise, just clear the space.
                (#Hierarchy (dec h-size) (vacant! idx h-array)))
              ## But if the sub-removal yielded a non-empty node, then
              ## just update the hiearchy branch.
              (#Hierarchy h-size (update! idx sub-node' h-array)))))))

    ## For #Base nodes, check whether the BitPosition is set.
    (#Base bitmap base)
    (let [bit (bit-position level hash)]
      (if (bit-position-is-set? bit bitmap)
        (let [idx (base-index bit bitmap)]
          (case (array.read idx base)
            #.None
            (undefined)

            ## If set, check if it's a sub-node, and remove the KV
            ## from it.
            (#.Some (#.Left sub-node))
            (let [sub-node' (remove' (level-up level) hash key Hash<k> sub-node)]
              ## Verify that it was removed.
              (if (is? sub-node sub-node')
                ## If not, there's also nothing to change here.
                node
                ## But if it came out empty...
                (if (empty?' sub-node')
                  ### ... figure out whether that's the only position left.
                  (if (only-bit-position? bit bitmap)
                    ## If so, removing it leaves this node empty too.
                    empty
                    ## But if not, then just unset the position and
                    ## remove the node.
                    (#Base (unset-bit-position bit bitmap)
                           (remove! idx base)))
                  ## But, if it did not come out empty, then the
                  ## position is kept, and the node gets updated.
                  (#Base bitmap
                         (update! idx (#.Left sub-node') base)))))

            ## If, however, there was a KV-pair instead of a sub-node.
            (#.Some (#.Right [key' val']))
            ## Check if the keys match.
            (if (:: Hash<k> = key key')
              ## If so, remove the KV-pair and unset the BitPosition.
              (#Base (unset-bit-position bit bitmap)
                     (remove! idx base))
              ## Otherwise, there's nothing to remove.
              node)))
        ## If the BitPosition is not set, there's nothing to remove.
        node))

    ## For #Collisions nodes, It need to find out if the key already existst.
    (#Collisions _hash _colls)
    (case (collision-index Hash<k> key _colls)
      ## If not, then there's nothing to remove.
      #.None
      node

      ## But if so, then check the size of the collisions list.
      (#.Some idx)
      (if (n.= 1 (array.size _colls))
        ## If there's only one left, then removing it leaves us with
        ## an empty node.
        empty
        ## Otherwise, just shrink the array by removing the KV-pair.
        (#Collisions _hash (remove! idx _colls))))
    ))

(def: (get' level hash key Hash<k> node)
  (All [k v] (-> Level Hash-Code k (Hash k) (Node k v) (Maybe v)))
  (case node
    ## For #Hierarchy nodes, just look-up the key on its children.
    (#Hierarchy _size hierarchy)
    (case (array.read (level-index level hash) hierarchy)
      #.None            #.None
      (#.Some sub-node) (get' (level-up level) hash key Hash<k> sub-node))

    ## For #Base nodes, check the leaves, and recursively check the branches.
    (#Base bitmap base)
    (let [bit (bit-position level hash)]
      (if (bit-position-is-set? bit bitmap)
        (case (array.read (base-index bit bitmap) base)
          #.None
          (undefined)
          
          (#.Some (#.Left sub-node))
          (get' (level-up level) hash key Hash<k> sub-node)

          (#.Some (#.Right [key' val']))
          (if (:: Hash<k> = key key')
            (#.Some val')
            #.None))
        #.None))

    ## For #Collisions nodes, do a linear scan of all the known KV-pairs.
    (#Collisions _hash _colls)
    (:: maybe.monad map product.right
        (array.find (|>> product.left (:: Hash<k> = key))
                    _colls))
    ))

(def: (size' node)
  (All [k v] (-> (Node k v) Nat))
  (case node
    (#Hierarchy _size hierarchy)
    (array@fold n.+ 0 (array@map size' hierarchy))
    
    (#Base _ base)
    (array@fold n.+ 0 (array@map (function (_ sub-node')
                                   (case sub-node'
                                     (#.Left sub-node) (size' sub-node)
                                     (#.Right _)       1))
                                 base))

    (#Collisions hash colls)
    (array.size colls)
    ))

(def: (entries' node)
  (All [k v] (-> (Node k v) (List [k v])))
  (case node
    (#Hierarchy _size hierarchy)
    (array@fold (function (_ sub-node tail) (list@compose (entries' sub-node) tail))
                #.Nil
                hierarchy)

    (#Base bitmap base)
    (array@fold (function (_ branch tail)
                  (case branch
                    (#.Left sub-node)
                    (list@compose (entries' sub-node) tail)

                    (#.Right [key' val'])
                    (#.Cons [key' val'] tail)))
                #.Nil
                base)
    
    (#Collisions hash colls)
    (array@fold (function (_ [key' val'] tail) (#.Cons [key' val'] tail))
                #.Nil
                colls)))

(type: #export (Dictionary k v)
  {#.doc "A dictionary implemented as a Hash-Array Mapped Trie (HAMT)."}
  {#hash (Hash k)
   #root (Node k v)})

(def: #export (new Hash<k>)
  (All [k v] (-> (Hash k) (Dictionary k v)))
  {#hash Hash<k>
   #root empty})

(def: #export (put key val dict)
  (All [k v] (-> k v (Dictionary k v) (Dictionary k v)))
  (let [[Hash<k> node] dict]
    [Hash<k> (put' root-level (:: Hash<k> hash key) key val Hash<k> node)]))

(def: #export (remove key dict)
  (All [k v] (-> k (Dictionary k v) (Dictionary k v)))
  (let [[Hash<k> node] dict]
    [Hash<k> (remove' root-level (:: Hash<k> hash key) key Hash<k> node)]))

(def: #export (get key dict)
  (All [k v] (-> k (Dictionary k v) (Maybe v)))
  (let [[Hash<k> node] dict]
    (get' root-level (:: Hash<k> hash key) key Hash<k> node)))

(def: #export (contains? key dict)
  (All [k v] (-> k (Dictionary k v) Bit))
  (case (get key dict)
    #.None     #0
    (#.Some _) #1))

(exception: #export key-already-exists)

(def: #export (try-put key val dict)
  {#.doc "Only puts the KV-pair if the key is not already present."}
  (All [k v] (-> k v (Dictionary k v) (Try (Dictionary k v))))
  (case (get key dict)
    #.None     (#try.Success (put key val dict))
    (#.Some _) (exception.throw ..key-already-exists [])))

(def: #export (update key f dict)
  {#.doc "Transforms the value located at key (if available), using the given function."}
  (All [k v] (-> k (-> v v) (Dictionary k v) (Dictionary k v)))
  (case (get key dict)
    #.None
    dict

    (#.Some val)
    (put key (f val) dict)))

(def: #export (upsert key default f dict)
  {#.doc (doc "Updates the value at the key; if it exists."
              "Otherwise, puts a value by applying the function to a default.")}
  (All [k v] (-> k v (-> v v) (Dictionary k v) (Dictionary k v)))
  (..put key
         (f (maybe.default default
                           (..get key dict)))
         dict))

(def: #export size
  (All [k v] (-> (Dictionary k v) Nat))
  (|>> product.right ..size'))

(def: #export empty?
  (All [k v] (-> (Dictionary k v) Bit))
  (|>> size (n.= 0)))

(def: #export (entries dict)
  (All [k v] (-> (Dictionary k v) (List [k v])))
  (entries' (product.right dict)))

(def: #export (from-list Hash<k> kvs)
  (All [k v] (-> (Hash k) (List [k v]) (Dictionary k v)))
  (list@fold (function (_ [k v] dict)
               (put k v dict))
             (new Hash<k>)
             kvs))

(template [<name> <elem-type> <side>]
  [(def: #export (<name> dict)
     (All [k v] (-> (Dictionary k v) (List <elem-type>)))
     (|> dict entries (list@map <side>)))]

  [keys   k product.left]
  [values v product.right]
  )

(def: #export (merge dict2 dict1)
  {#.doc (doc "Merges 2 dictionaries."
              "If any collisions with keys occur, the values of dict2 will overwrite those of dict1.")}
  (All [k v] (-> (Dictionary k v) (Dictionary k v) (Dictionary k v)))
  (list@fold (function (_ [key val] dict) (put key val dict))
             dict1
             (entries dict2)))

(def: #export (merge-with f dict2 dict1)
  {#.doc (doc "Merges 2 dictionaries."
              "If any collisions with keys occur, a new value will be computed by applying 'f' to the values of dict2 and dict1.")}
  (All [k v] (-> (-> v v v) (Dictionary k v) (Dictionary k v) (Dictionary k v)))
  (list@fold (function (_ [key val2] dict)
               (case (get key dict)
                 #.None
                 (put key val2 dict)

                 (#.Some val1)
                 (put key (f val2 val1) dict)))
             dict1
             (entries dict2)))

(def: #export (re-bind from-key to-key dict)
  (All [k v] (-> k k (Dictionary k v) (Dictionary k v)))
  (case (get from-key dict)
    #.None
    dict

    (#.Some val)
    (|> dict
        (remove from-key)
        (put to-key val))))

(def: #export (select keys dict)
  {#.doc "Creates a sub-set of the given dict, with only the specified keys."}
  (All [k v] (-> (List k) (Dictionary k v) (Dictionary k v)))
  (let [[Hash<k> _] dict]
    (list@fold (function (_ key new-dict)
                 (case (get key dict)
                   #.None       new-dict
                   (#.Some val) (put key val new-dict)))
               (new Hash<k>)
               keys)))

(structure: #export (equivalence (^open ",@."))
  (All [k v] (-> (Equivalence v) (Equivalence (Dictionary k v))))
  
  (def: (= test subject)
    (and (n.= (size test)
              (size subject))
         (list.every? (function (_ k)
                        (case [(get k test) (get k subject)]
                          [(#.Some tk) (#.Some sk)]
                          (,@= tk sk)

                          _
                          #0))
                      (keys test)))))

(structure: functor'
  (All [k] (Functor (Node k)))
  
  (def: (map f fa)
    (case fa
      (#Hierarchy size hierarchy)
      (#Hierarchy size (array@map (map f) hierarchy))
      
      (#Base bitmap base)
      (#Base bitmap (array@map (function (_ either)
                                 (case either
                                   (#.Left fa')
                                   (#.Left (map f fa'))
                                   
                                   (#.Right [k v])
                                   (#.Right [k (f v)])))
                               base))
      
      (#Collisions hash collisions)
      (#Collisions hash (array@map (function (_ [k v])
                                     [k (f v)])
                                   collisions)))))

(structure: #export functor
  (All [k] (Functor (Dictionary k)))
  
  (def: (map f fa)
    (update@ #root (:: ..functor' map f) fa)))