Pairs

Pair predicates work with lists of two-element lists [Key, Value], providing key-value processing operations. For proper key-value mappings with unification support, see Dicts & Sets.

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Quick Example

Test("unzip") <- (
    pairs_keys_values([[1, "a"], [2, "b"], [3, "c"]], KEYS, VALUES),
    KEYS == [1, 2, 3],
    VALUES == ["a", "b", "c"]
)

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Predicates

pairs_keys_values/3

pairs_keys_values(Pairs, Keys, Values) — bidirectional conversion between a list of pairs and separate key/value lists.

Decompose mode (Pairs bound):

Test("decompose") <- (
    pairs_keys_values([["name", "alice"], ["age", 30]], KS, VS),
    KS == ["name", "age"],
    VS == ["alice", 30]
)

Construct mode (Keys + Values bound):

Test("construct") <- (
    pairs_keys_values(PAIRS, ["x", "y", "z"], [1, 2, 3]),
    PAIRS == [["x", 1], ["y", 2], ["z", 3]]
)

Keys and Values must have equal length when constructing.

pairs_keys/2

pairs_keys(Pairs, Keys) — extract the first element from each pair.

Test("keys") <- pairs_keys([[1, "a"], [2, "b"], [3, "c"]], [1, 2, 3])

pairs_values/2

pairs_values(Pairs, Values) — extract the second element from each pair.

Test("values") <- pairs_values([[1, "a"], [2, "b"], [3, "c"]], ["a", "b", "c"])

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Patterns & Recipes

Lookup by key

lookup(KEY, PAIRS, VALUE) <- in_([KEY, VALUE], PAIRS)

Test("lookup") <- lookup("b", [["a", 1], ["b", 2], ["c", 3]], 2)

sort pairs by key

Test("sort by key") <- (
    sort_by((P, K) <- list_item(0, P, K), [["b", 2], ["a", 1], ["c", 3]], SORTED),
    pairs_keys(SORTED, ["a", "b", "c"])
)

Invert a mapping (using maplist)

swap_pair([K, V], [V, K]),

invert(PAIRS, INVERTED) <- (
    maplist(swap_pair, PAIRS, INVERTED)
)

Test("invert") <- invert([["a", 1], ["b", 2]], [[1, "a"], [2, "b"]])

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See also: Lists — general list operations, Dicts & Sets — DictTerm for proper key-value mapping, Higher-Order — maplist, sort_by for pair processing.