08-maps.md

8. Maps

Goop has a first-class map[K] V type that lowers to a Go map[K]V. Use the prelude Map.* helpers — no @[go] required for ordinary table lookups.

See STYLE.md and 29-maps.md.

Create and update

let table : map[string] int = Map.make () in
let _ = Map.add table "foo" 1 in
let _ = Map.add table "bar" 2 in
()

Maps are Go reference types: Map.add / Map.remove mutate in place and return unit.

Lookup

match Map.get table "foo" with
| Some n -> println (int_to_string n)
| None -> println "missing"

Binding	Type	Notes
Map.make	unit -> map['k] 'v	Empty map
Map.get	map['k] 'v -> 'k -> 'v option	Missing key → None
Map.add	map['k] 'v -> 'k -> 'v -> unit	Insert / overwrite
Map.remove	map['k] 'v -> 'k -> unit	Delete key
Map.mem	map['k] 'v -> 'k -> bool	Membership
Map.size	map['k] 'v -> int	Entry count

Type spelling

• Write map[K] V (Go-like brackets for the key; value follows).
• Arrows bind outside: map[string] int -> unit means (map[string] int) -> unit.
• Function-valued maps need parentheses: map[string] (int -> int).

Optional std.map import

import goop . "std.map"

let m = make ()   (* same as Map.make *)

Runnable example

See maps.goop:

./goop check docs/examples/maps.goop

Common errors

Message	Cause	Fix
type mismatch on Map.get	Wrong key or value type	Annotate map[K] V or check call args
Unexpected parse around ->	Function-valued map without parens	Use map[string] (int -> int)
Unbound Map	Typo / shadowed name	Prelude Map is always in scope

Full catalog: error reference.

Next steps

• Go / C interop — maps also appear in Go stubs
• Writing tools — files + maps via import go
• Maps design