gossamer/map

JavaScript Map bindings for interop with APIs that produce or consume a Map. Treated as a transit type: bridge to gleam/dict.Dict via to_dict and operate on the canonical Gleam surface for transformations, then from_dict back when handing off to JavaScript. The non-mutating reads (size, get, has, keys, values, entries) stay for one-shot interop without round-tripping through Dict.

Types

Map

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A JavaScript Map, holding key-value pairs and preserving insertion order. Supports any key type (unlike plain objects).

For most Gleam use cases, prefer gleam/dict.Dict. This binding exists for interop with JavaScript code that expects a Map; bridge with to_dict / from_dict and operate on the Dict surface for transformations.

Object keys (records, lists, tuples) are matched by JavaScript reference identity, not by Gleam value equality — two equal-by-value tuples constructed separately are distinct keys. Primitive keys (Int, Float, String, Bool) use value equality.

See Map on MDN.

pub type Map(key, value)

Values

entries

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pub fn entries(
  map: Map(key, value),
) -> yielder.Yielder(#(key, value))

Returns the #(key, value) pairs of the Map in insertion order.

from_dict

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pub fn from_dict(dict: dict.Dict(key, value)) -> Map(key, value)

Creates a Map from a Dict. Iteration order follows the Dict’s to_list order.

from_list

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pub fn from_list(entries: List(#(key, value))) -> Map(key, value)

Creates a Map from a list of key-value pairs. Later pairs override earlier ones for the same key.

get

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pub fn get(
  from map: Map(key, value),
  key key: key,
) -> Result(value, Nil)

Returns the value associated with the given key, or Error(Nil) if the key is absent.

has

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pub fn has(in map: Map(key, value), key key: key) -> Bool

Returns whether the Map contains the given key.

keys

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pub fn keys(map: Map(key, value)) -> yielder.Yielder(key)

Returns the keys of the Map in insertion order.

new

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pub fn new() -> Map(key, value)

Creates an empty Map.

size

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pub fn size(map: Map(key, value)) -> Int

The number of entries in the Map.

to_dict

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pub fn to_dict(map: Map(key, value)) -> dict.Dict(key, value)

Converts the Map to a Dict. Iteration order is preserved as Dict insertion order.

values

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pub fn values(map: Map(key, value)) -> yielder.Yielder(value)

Returns the values of the Map in insertion order.