Expressions

Blocks

A block is zero or more statements and a final expression:

{
    let x = 1;
    <(x, 2) | add
}

A semicolon follows every statement except the block's last expression. The last expression is the block's value. An empty block is (,). A block whose last expression is a cut is a command.

A chain ends at the ; or } of the block, and at the , or ) that closes a component. A component may itself be a chain that opens with <.

let

let pattern = expression;
let+ pattern = expression;
let- pattern = expression;

The pattern is irrefutable. let+ and let- are written with the sign touching let. Polarity is Polarity.

of

of scrutinee {
    pattern => expression,
    pattern => expression,
}

An arm is a pattern, then => for data or <= for a demand, then a body. Arms cover the scrutinee exactly once. There is no guard between the pattern and the arrow. A test is an of in the arm. The scrutinee's braces, when it has them, are parsed as the of arms, so a record scrutinee is parenthesized or bound with let first.

An arm that cuts has type (;) and does not fix the type of the of.

mu

mu Type { pattern => command, … }
mu Type { copattern <= command, … }
mu Type { name <= command }
mu Type {}

=> builds a consumer of Type. <= with a copattern answers a menu or a form. <= with one name captures the surrounding continuation and produces Type. Empty braces are the consumer of a type that has no values.

A menu copattern follows the field. item: out <= body binds the continuation. item(param): out <= body binds the arguments and then the continuation. item(param) <= body binds the continuation under the field's name.

A mu arm that matches data ends in a command: a cut to a consumer in scope.

Calls and constructors

A named function is applied by flow: <argument | function. Several parameters are one group: <(a, b) | function. A nullary function is function() or <(,) | function.

A constructor keeps its parentheses: Cons(head, tail), Point { x: 1, y: 2 }. A variant with no payload is the name alone. An anonymous alternative is ::0(value), counting from zero.

A menu demand is menu.field for the function, menu.field(arguments) for the call, or <arguments | menu.field. A record or tuple projection is value.field and value.0.

do, hn, and reset

do expression handler
hn Effect { clauses }
hn [Effect, Effect] { clauses }
hn { clauses }
reset expression

handler is an hn expression, a name bound to one, or a hand. The body of do is one expression, so a chain is parenthesized or written as a block. Clause syntax is Effects and handlers.

reset expression installs a handler with no clauses. It bounds jumps and discharges nothing.

Chains

Chain forms are the whole of Flow. Inside a chain, name => expression names the value flowing in, and name <= expression names the consumer the rest of the chain builds. A binder's body is one stage and ends at the next |.