type-hole-union

The _ hole infers one inhabitant type; the ∪ hole infers a finite union; T ∪ none stays a single optional inhabitant.

Syntax: const _ x ← value; const ∪ x ← a; x ← b; const list<∪> xs ← [a, b]

Category#

types

Related#

• unio
• null

Examples#

radix/corpus/type-hole-union/type-hole-union.fab (canonical · type)#

The _ hole infers one inhabitant type; the ∪ hole infers a finite union; T ∪ none stays a single optional inhabitant.

# =============================================================================
# type-hole-union — two type holes (`_` vs `∪`) and the nullable exception
# =============================================================================
#
# What this teaches:
#   • `_` is the monomorphic hole: it infers exactly ONE inhabitant type.
#     `const _ count ← 3` solves `count : int`.
#   • `∪` is the union hole: the slot accumulates a member type from every
#     direct flow. `var ∪ mixed ← 1` then `mixed ← "salve"` solves
#     `mixed : int ∪ string` without authoring the member list by hand.
#   • `list<∪>` learns heterogeneous element types the same way:
#     `list<∪> nodes ← [1, "two", 3]` solves to `list<int ∪ string>`.
#   • `T ∪ null` (canonical `T ∪ none`) is a single OPTIONAL inhabitant,
#     not a multi-member sum — a `_` hole solved to it emits no shape
#     warning.
#
# Cross-shape solves still typecheck but are diagnosed: `_` solved to a
# multi-member union emits WARN016 (`monomorphic_hole_solved_to_union`) and
# `∪` solved to a single type emits WARN017 (`union_hole_solved_monomorphically`).
# Both are promotable through the existing deny machinery; this exemplum shows
# only the clean shapes.
#
# The consumer functions below only exist to use the union-typed slots so the
# exemplum checks warning-free; the type lesson is in the annotations.
#
# See also: unio, none
# =============================================================================

# `_` infers exactly one inhabitant type from the initializer: `n` is `int`.
fn member_count(list<int ∪ string> xs) → int {
    const _ n ← xs.length()
    if n ≻ 0 then return 1
    return 0
}

# `∪` accumulates a member per direct flow; `match` type arms name the
# members, and an `if` inside an arm tests the value.
fn consume(int ∪ string value) → int {
    match value {
        case int const n {
            if n ≡ 7 then return 1
        }
        case string const s {
            if s ≡ "x" then return 2
        }
    }
    return 0
}

# `T ∪ null` is one optional inhabitant, so a `null` arm is the null case.
fn maybe_len(string ∪ none value) → int {
    match value {
        case none { return 0 }
        default { return 1 }
    }
}

test "the two hole kinds infer cleanly without shape warnings" {
    # `_`: exactly one inhabitant type.
    const _ count ← 3

    # `∪`: every direct flow adds a member — solves to `int ∪ string`.
    var ∪ mixed ← 1
    mixed ← "salve"

    # `list<∪>`: heterogeneous elements solve to `list<int ∪ string>`.
    const list<∪> nodes ← [1, "two", 3]

    # `T ∪ null`: one optional inhabitant — `_` over it stays quiet.
    const string ∪ none opt ← "text"
    const _ maybe ← opt

    const _ m ← member_count(nodes)
    const _ n ← consume(mixed)
    const _ o ← maybe_len(maybe)

    assert count ≡ 3
    assert m ≡ 1
    assert n ≡ 0
    assert o ≡ 1
}

# Runnable path for the MIR stepper (S5-U4): same union surface, printed
# so the script harness has an exact stdout oracle (`type-hole-union.expected`).
main {
    # `_`: exactly one inhabitant type.
    const _ count ← 3

    # `∪`: every direct flow adds a member — solves to `int ∪ string`.
    var ∪ mixed ← 1
    mixed ← "salve"

    # `list<∪>`: heterogeneous elements solve to `list<int ∪ string>`.
    const list<∪> nodes ← [1, "two", 3]

    # `T ∪ null`: one optional inhabitant — `_` over it stays quiet.
    const string ∪ none opt ← "text"
    const _ maybe ← opt

    const _ m ← member_count(nodes)
    const _ n ← consume(mixed)
    const _ o ← maybe_len(maybe)

    assert count ≡ 3
    assert m ≡ 1
    assert n ≡ 0
    assert o ≡ 1

    print count
    print m
    print n
    print o
}

Expected output:

3
1
0
1