Session Types Guide

This guide is the user-facing reference for stdlib/schan.tur and the broader session-types story; the design record is the archived stdlib-session-typed-channels-plan.

Turmeric supports session types -- a type discipline that statically verifies communication protocols between concurrent processes. The feature is enabled by default; no compiler flag is required.

Binary Session Types (SS0-SS4)

Binary session types describe a two-party communication channel. One end of the channel runs a protocol P; the other end runs the dual protocol dual(P).

make-session, send, recv, close

;; Allocate a two-ended channel for the protocol Send<int, Close>.
(let [[a b] (make-session (Send int Close))]
  ...)

;; Allocate a two-ended channel for the protocol Send<int, Close>.
let [[a b] make-session((Send int Close))]
  ...

The pair [a b] gives both endpoints. Endpoint a has type Session[Send int Close]; endpoint b has the dual type Session[Recv int Close].

send -- advance the sender side by one message:

(let [a (send a 42)]  ; a advances from Send<int,Close> to Close
  (close a))

let [a send(a 42)]  ; a advances from Send<int,Close> to Close
  close(a)

recv -- advance the receiver side by one message; returns [value new-ch]:

(let [[v b] (recv b)]  ; v = 42, b advances from Recv<int,Close> to Close
  (close b))

let [[v b] recv(b)]  ; v = 42, b advances from Recv<int,Close> to Close
  close(b)

close -- consume the channel after the protocol ends (type Close).

Choice: choose-left, choose-right, offer

When the sender can choose between two branches use choose-left / choose-right; the receiver uses offer and pattern-matches on Left/Right:

;; Sender side: Choose<Send int Close, Close>
(let [ch (choose-left ch)]  ; picks the Left branch
  (let [ch (send ch 7)]
    (close ch)))

;; Receiver side: Branch<Recv int Close, Close>
(match (offer ch)
  (Left ch)
    (let [[n ch] (recv ch)]
      (close ch))
  (Right ch)
    (close ch))

;; Sender side: Choose<Send int Close, Close>
let [ch choose-left(ch)]  ; picks the Left branch
  let [ch send(ch 7)]
    close(ch)

;; Receiver side: Branch<Recv int Close, Close>
match offer(ch)
  (Left ch)
  let [[n ch] recv(ch)]
    close(ch)
  (Right ch)
  close(ch)

Recursive protocols: Rec

Rec introduces a recursive protocol variable self that unrolls on each loop iteration:

;; Server: repeat (recv int, send int) until client closes
(defn echo-server [^linear ch :(Session (Rec self (Branch (Recv int (Send int self)) Close)))] : nil
  (match (offer ch)
    (Left ch)
      (let [[n ch] (recv ch)]
        (let [ch (send ch n)]
          (echo-server ch)))
    (Right ch)
      (close ch)))

;; Server: repeat (recv int, send int) until client closes
defn echo-server [^linear ch :(Session (Rec self (Branch (Recv int (Send int self)) Close)))] :nil
  match offer(ch)
    (Left ch)
    let [[n ch] recv(ch)]
      let [ch send(ch n)]
        echo-server(ch)
    (Right ch)
    close(ch)

See stdlib/session.tur for the echo-server-loop and echo-client-call helpers that wrap this pattern.

Timeouts

recv-timeout is a non-blocking receive that returns a Choice value:

(match (recv-timeout ch 500)  ; 500 ms deadline
  (Left [v ch]) (do (println v) (close ch))
  (Right ch)    (do (println "timed out") (close ch)))

match recv-timeout(ch 500)  ; 500 ms deadline
  (Left [v ch])
  do
    println(v)
    close(ch)
  (Right ch)
  do
    println("timed out")
    close(ch)

Duality

The duality rule governs how the two ends of a channel relate:

make-session accepts one protocol type and automatically produces the dual for the second endpoint.

Effect integration

Session channels and algebraic effects can coexist freely. A function may perform effects while holding a linear channel, as long as the channel is consumed exactly once along every code path:

(defeffect Log [msg :cstr] :nil)

(defn logged-send [^linear ch :(Session (Send int Close)) val : int] : int
  (perform (Log "before send"))
  (let [ch (send ch val)]
    (close ch)
    0))

defeffect Log [msg :cstr] :nil

defn logged-send [^linear ch :(Session (Send int Close)) val :int] :int
  perform(Log("before send"))
  let [ch send(ch val)]
    close(ch)
    0

See tests/fixtures/session-effects/ for a complete example.

Multi-Party Session Types (SS5-SS8)

Multi-party session types generalize binary sessions to N >= 2 participants. A global protocol specifies all interactions between named roles; the compiler projects it onto each role's local view.

defprotocol

Declare a global protocol with defprotocol:

(defprotocol Ping [A B]
  (-> A B int)    ; A sends an int to B
  (-> B A int))   ; B replies with an int to A

defprotocol Ping [A B]
  (-> A B int)    ; A sends an int to B
  (-> B A int)    ; B replies with an int to A

The role list [A B] names each participant. Each (-> From To type) line is one message transfer.

make-protocol, send-to, recv-from, close

After declaring a protocol, allocate role endpoints with make-protocol:

(let [[ra rb] (make-protocol Ping)]
  ...)

let [[ra rb] make-protocol(Ping)]
  ...

ra has type (Role Ping A) and rb has type (Role Ping B). Each role endpoint is linear -- it must be consumed exactly once.

send-to -- send a message to a named role peer:

(let [ra (send-to ra B 42)]  ; A sends 42 to B; ra advances in protocol
  ...)

let [ra send-to(ra B 42)]  ; A sends 42 to B; ra advances in protocol
  ...

recv-from -- receive a message from a named role peer; returns [value new-ch]:

(let [[v rb] (recv-from rb A)]  ; B receives from A; v = 42
  ...)

let [[v rb] recv-from(rb A)]  ; B receives from A; v = 42
  ...

close -- close the role endpoint after the protocol is complete:

(close ra)

close(ra)

Full two-role ping example:

(defprotocol Ping [A B]
  (-> A B int)
  (-> B A int))

(defn role-a [^linear ch :(Role Ping A)] : nil
  (let [ch (send-to ch B 42)]
    (let [[v ch] (recv-from ch B)]
      (println v)
      (close ch))))

(defn role-b [^linear ch :(Role Ping B)] : nil
  (let [[v ch] (recv-from ch A)]
    (let [ch (send-to ch A v)]
      (close ch))))

(defn main [] : int
  (let [[ra rb] (make-protocol Ping)]
    (let [t (spawn (fn [] (role-b rb)))]
      (role-a ra)
      (join t)))
  0)

defprotocol Ping [A B]
  (-> A B int)
  (-> B A int)

defn role-a [^linear ch :(Role Ping A)] :nil
  let [ch send-to(ch B 42)]
    let [[v ch] recv-from(ch B)]
      println(v)
      close(ch)

defn role-b [^linear ch :(Role Ping B)] :nil
  let [[v ch] recv-from(ch A)]
    let [ch send-to(ch A v)]
      close(ch)

defn main [] :int
  let [[ra rb] make-protocol(Ping)]
    let [t spawn(fn([] role-b(rb)))]
      role-a(ra)
      join(t)
  0

Three or more roles

defprotocol and make-protocol support any number of roles (N >= 2). For a three-role pipeline:

(defprotocol Pipeline [A B C]
  (-> A B int)   ; A sends to B
  (-> B C int))  ; B forwards to C

(defn main [] : int
  (let [[ra rb rc] (make-protocol Pipeline)]
    (let [ta (spawn (fn [] (role-a ra)))]
      (let [tb (spawn (fn [] (role-b rb)))]
        (role-c rc)
        (join ta)
        (join tb))))
  0)

defprotocol Pipeline [A B C]
  (-> A B int)   ; A sends to B
  (-> B C int)   ; B forwards to C

defn main [] :int
  let [[ra rb rc] make-protocol(Pipeline)]
    let [ta spawn(fn([] role-a(ra)))]
      let [tb spawn(fn([] role-b(rb)))]
        role-c(rc)
        join(ta)
        join(tb)
  0

The runtime uses a shared router so all N role endpoints communicate through a single lock-based message router allocated on the heap.

Projection algorithm

At compile time the elaborator projects the global protocol onto each role's local view. Projection removes all interactions that do not involve the current role:

• A message (-> From To T) projects to Send T rest for From, and Recv T rest for To. For any other role R, it is transparent (role R keeps its own remaining protocol).
• Choice branches that a role does not participate in must be uniform across branches (mergeability condition). If they are not, the compiler emits TUR-E0220.

The projection check happens in elab_global.c when (make-protocol P) is elaborated.

Error Codes

Session-Typed Channel Wrappers (stdlib/schan.tur)

The built-in session types above are their own typed-channel runtime. When you instead want to put a protocol discipline over an ordinary buffered channel -- e.g. a worker-pool request/response, or an RPC pipe -- stdlib/schan.tur provides a thin generic wrapper, SChan<p>, that carries a protocol phantom p advanced by each operation:

(import schan :refer [SChan SSend SRecv SClose
                      schan-new schan-send schan-recv schan-close])

The phantom is built from three type-level tags (the session-type names Send / Recv / Close are reserved primitive constructors, so the wrapper uses the S-prefixed spellings):

Each operation consumes the channel at one protocol state and returns it at the next, so the phantom is threaded through the result type:

schan-send  : SChan<SSend T R> -> T  -> SChan<R>
schan-recv  : SChan<SRecv T R>       -> Pair<T SChan<R>>
schan-close : SChan<SClose>          -> nil

A round trip of SSend int (SRecv int SClose):

(let [c0 (:: (schan-new 2) (SChan (SSend int (SRecv int SClose))))
      c1 (schan-send c0 7)      ;; c1 : SChan<SRecv int SClose>
      p  (schan-recv c1)        ;; p  : Pair<int SChan<SClose>>
      v  (pair-fst p)           ;; v  : int  (= 7)
      c2 (pair-snd p)]          ;; c2 : SChan<SClose>
  (schan-close c2))

Because the phantom advances with every step, skipping or reordering a step is a compile-time type error. Calling schan-close while the channel is still at SChan<SRecv int SClose> fails with a TUR-E0001 phantom mismatch:

error [TUR-E0001]: function 'schan-close' arg 1:
  expected (type-app SChan SClose),
  got (type-app SChan (type-app (type-app SRecv int) SClose))

SChan is :linear, so the protocol additionally cannot be replayed -- each step consumes its handle exactly once.

Runnable examples: tests/fixtures/schan-roundtrip (single round trip), tests/fixtures/schan-worker-pool (a request/response served by worker threads reading from the wrapped channel), and tests/fixtures/errors/schan-skip-step (the phantom-mismatch failure). The wrapper sits on top of the low-level tur/chan channels, which keep their untyped surface for callers that do not want the protocol discipline.

schan-recv used to take a cell. Until 2026-08-20 it returned only the continuation and wrote the received value through a caller-allocated out-parameter (schan-cell-new / -get / -free, all now removed). That was a workaround for a monomorphizer defect -- a generic function could not return a parametric aggregate whose element type is a phantom carried inside an opaque argument. The defect was fixed 2026-06-05 (docs/archive/history/generic-struct-opaque-element-miscompile.md), so the Pair<T SChan<R>> signature the design always wanted is what ships now.

Getting More Help

Use tur explain to get a detailed description of any error code:

tur explain TUR-E0212
tur explain TUR-E0220

The test fixtures under tests/fixtures/session-* and tests/fixtures/errors/session-* provide runnable examples for every feature and failure mode described in this guide.