channel

A channel closes itself when its conversation completes; a pool of many keeps only the unfinished ones open, and close() ends one early and reports its error.

Syntax: const bool clausa ← s.close()

Category#

endpoint

Related#

• call
• sermo
• scrinium
• send
• recv
• cursor

Examples#

radix/corpus/ad/sermo-channel.fab (canonical · type)#

English reader names for the conversation family: channel, frame, send<T>, recv<T>

# =============================================================================
# channel — the conversation family under the English reader surface.
# =============================================================================
#
# What this teaches:
#   • `channel` is the en spelling of `channel`, the handle `call` returns
#   • `frame<T>` is the en spelling of `frame<T>`, one typed frame
#   • `s.send<T>()` / `s.recv<T>()` are the en spellings of the outbound
#     `send<T>` and inbound `recv<T>` views
#
# Common mistakes:
#   • treating the words as reserved: `channel`, `frame`, `send` and `recv`
#     are type names only, and stay ordinary identifiers elsewhere
#
# See also: call, channel, frame, send, recv, cursor
# =============================================================================

fn count_items(channel s) → int {
    var int n ← 0
    for from s.recv<string>().cursor() const item {
        print item.data
        n ← n + 1
    }
    return n
}

main {
    const channel s ← call 'runtime:echo' ("salve")
    const send<value> outbound ← s.send<value>()
    outbound.da("ping")
    outbound.fini()
    print count_items(s)

    const channel second ← call 'runtime:echo' ("vale")
    const frame<string> ∪ none first ← second.recv<string>().accipe()
    print first?.data
}

Expected output:

salve
1
vale

radix/corpus/ad/sermo-close.fab (canonical · method)#

A channel closes itself when its conversation completes; a pool of many keeps only the unfinished ones open, and close() ends one early and reports its error.

# =============================================================================
# channel — a conversation closes itself when it completes; `close()` is early.
# =============================================================================
#
# What this teaches:
#   • Completion closes — when the handler returns (or fails), the channel
#     releases its handler task and router slot at once, even while its
#     handle still sits in a map; the handle keeps reading as its result
#   • A pool of many — finished conversations cost nothing while the pool
#     waits on the rest, so a map of many sermones is cheap
#   • Explicit close — `s.close()` ends a conversation early (a live handler
#     is cancelled, unread frames are dropped) and reports the error the
#     conversation ended with through `⇥ string`; it returns `true` when
#     this call closed the handle
#   • Last reference — a channel nobody can reach is closed as a safety net,
#     but errors from that automatic close are lost; call `close()` when the
#     error matters
#
# Common mistakes:
#   • expecting close() to wait — a handler still running is cancelled, so
#     read to the end first when its outcome matters
#   • keeping a channel inside a reference cycle — a cycle never reaches its
#     last reference, so the compiler warns (WARN031) and it never
#     auto-closes; `close()` it explicitly
#
# See also: call, channel, recv, do, catch
# =============================================================================

@ call 'numeri:bis'
fn bis(int n) → int {
    return n * 2
}

@ call 'numeri:numera'
fn numera(int n) generator → int {
    for range 0‥n const i {
        yield i
    }
}

@ call 'numeri:defice'
fn defice(int n) → int ⇥ string {
    throw "defecit §"(n)
}

fn sum(list<int> partes) → int {
    var int totum ← 0
    for from partes const pars {
        totum ← totum + pars
    }
    return totum
}

main {
    # Four conversations finish at once; their handles wait in the pool.
    var map<int, channel> piscina ← empty
    for range 0‥4 const i {
        piscina[i] ← call 'numeri:bis' (i)
    }

    # A longer conversation streams while the finished ones hold nothing.
    const channel longus ← call 'numeri:numera' (5)
    for range 0‥4 const i {
        print piscina[i] ↦ int
    }
    print sum(longus ↦ list<int>)

    # Reading waits for the handler (a failed read defaults to 0); close()
    # then reports the error the conversation ended with.
    const channel float ← call 'numeri:defice' (7)
    print float ↦ int ⊥ 0
    do {
        const bool clausa ← fractus.close()
        print clausa
    } catch err {
        print err
    }

    # Closing early drops the unread reply; a live handler is cancelled.
    do {
        const channel ultimus ← call 'numeri:bis' (21)
        const bool clausa ← ultimus.close()
        print clausa
    } catch err {
        print err
    }
}

Expected output:

0
2
4
6
10
0
defecit 7
verum