This page describes the semantics of the code constructs whose grammar is given in grammar.md. The formation rules there are not named, and here they are identified by either the name of the term or by copying the rule entirely if there are several alternative productions.
Here we assume that the referent of each identifier, or equivalently the connections between identifiers, have been identified according to the scoping rules.
Evaluation is an ordered process, and any actions required to evaluate a node always have a specified order unless performing them in any order would have the same effect. Side effects that are relevant to ordering are setting and getting the value of a variable, and causing an error. Errors described in this page are "evaluation errors" and can be caught by the Catch (
⎊) modifier. If caught, evaluation halts without attempting to complete any in-progress node, and is restarted by Catch.
As specified, BQN programs can involve an arbitrary amount of information, but when run there will be memory and possibly other limitations. To accommodate this, any part of evaluation can cause an error, if a resource such as memory, stack memory, or limited execution time is exhausted.
The result of parsing a valid BQN program is a
PROGRAM, and the program is run by evaluating this term.
BODY is a list of
STMTs, which are evaluated in program order. A
BODY also allows an
EXPR followed by
"?" in place of an
STMT: then the expression is evaluated as usual but its result is checked as discussed below. A result is always required for
BODY nodes, and sometimes for
PROGRAM nodes (for example, when loaded with
•Import). If any identifiers in the node's scope are exported, or any of its statements is an
EXPORT, then the result is the namespace created in order to evaluate the node. If a result is required but the namespace case doesn't apply, then the last
STMT node must be an
EXPR and its result is used. The statement
EXPR evaluates some BQN code and possibly assigns the results, while
nothing evaluates any
Derv terms it contains but discards the results. An
EXPORT statement performs no action.
A block consists of several
BODY terms, some of which may have an accompanying header describing accepted inputs and how they are processed. An immediate block
brImm can only have one
BODY, and is evaluated by evaluating it. Other types of blocks don't evaluate any
BODY immediately, but instead return a function or modifier that obtains its result by evaluating a particular
BODY is identified and evaluated once the block has received enough inputs (operands or arguments), which for modifiers can take one or two calls: if two calls are required, then on the first call the operands are simply stored and no code is evaluated yet. The stored values can be accessed by equality checking, or
•Decompose if defined. Two calls are required if there is more than one
BODY term, if the
BODY contains the special names
𝕨𝕩𝕤𝕎𝕏𝕊, or if its header specifies arguments (the header-body combination is a
_cCase_). Otherwise only one is required.
To evaluate a block when enough inputs have been received, first the correct case must be identified. To do this, first each special case (
A_CASE nodes whose
"⁼", is checked in order to see if its arguments are strucurally compatible with the given arguments. That is, is
headW is an
lhs, there must be a left argument matching that structure, and if
headX is an
lhs, the right argument must match that structure. This means that
𝕨 not only matches any left argument but also no argument. The test for compatibility is the same as for multiple assignment described below, except that the header may contain constants, which must match the corresponding part of the given argument. If no special case matches, then an appropriate general
CASE is used: if there are two, the first is used with no left argument and the second with a left argument; if there are one, it is always used, and if there are none, an error results.
When a predicate
"?" is evaluated, it may change the choice of case. The associated
EXPR is evaluated and its result is checked. If it's not one of the numbers
1, an error results. If it's
1, evaluation of the
BODY continues as usual. If it's
0, evaluation is stopped and the next compatible
BODY term is evaluated using the block's original inputs.
Inputs and other names are bound when evaluation of a
BODY is begun. Special names are always bound when applicable:
𝕨𝕩𝕤 if arguments are used,
𝕨 if there is a left argument,
𝕗𝕘 if operands are used, and
_𝕣_ for modifiers and combinators, respectively. Any names in the header are also bound, allowing multiple assignment for arguments.
If there is no left argument, but the
𝕎 at the top level, then it is conceptually re-parsed with
𝕨 replaced by
· to give a monadic version before application; this modifies the syntax tree by replacing some instances of
nothing. The token
𝕎 is not allowed in this case and causes an error. Re-parsing
𝕨 can also cause an error if it's used as an operand or list element, where
nothing is not allowed by the grammar. Note that these errors must not appear if the block is always called with two arguments. True re-parsing is not required, as the same effect can also be achieved dynamically by treating
· as a value and checking for it during execution. If it's used as a left argument, then the function should instead be called with no left argument (and similarly in trains); if it's used as a right argument, then the function and its left argument are evaluated but rather than calling the function
· is "returned" immediately; and if it's used in another context then it causes an error.
An assignment is one of the four rules containing
ASGN. It is evaluated by first evaluating the right-hand-side
_m2Exp_ expression, and then storing the result in the left-hand-side identifier or identifiers. The result of the assignment expression is the result of its right-hand side. Except for subjects, only a lone identifier is allowed on the left-hand side and storage sets it equal to the result. For subjects, destructuring assignment is performed when an
lhsStr. Destructuring assignment is performed recursively by assigning right-hand-side values to the left-hand-side targets, with single-identifier assignment as the base case. The target
"·" is also possible in place of a
NAME, and performs no assignment.
The right-hand-side value, here called
v, in destructuring assignment must be a list (rank 1 array) or namespace. If it's a list, then each
LHS_ENTRY node must be an
LHS_ELT. The left-hand side is treated as a list of
lhs targets, and matched to
v element-wise, with an error if the two lists differ in length. If
v is a namespace, then the left-hand side must be an
lhsStr where every
LHS_ATOM is an
NAME, or an
lhsList where every
LHS_ENTRY is an
lhs "⇐" NAME, so that it can be considered a list of
NAME nodes some of which are also associated with
lhs nodes. To perform the assignment, the value of each name is obtained from the namespace
v, giving an error if
v does not define that name. The value is assigned to the
lhs node if present (which may be a destructuring assignment or simple subject assignment), and otherwise assigned to the same
NAME node used to get it from
Modified assignment is the subject assignment rule
lhs Derv "↩" subExpr?. In this case,
lhs is evaluated as if it were a
subExpr (the syntax is a subset of
subExpr), and passed as an argument to
Derv. The full application is
lhs Derv subExpr, if
subExpr is given, and
Derv lhs otherwise. Its value is assigned to
lhs, and is also the result of the modified assignment expression.
We now give rules for evaluating an
_mod2_ expression (the possible options for
ANY). A literal or primitive
_cl_ has a fixed value defined by the specification (literals and built-ins). An identifier
_c_, if not preceded by
atom ".", must have an associated variable due to the scoping rules, and returns this variable's value, or causes an error if it has not yet been set. If it is preceded by
atom ".", then the
atom node is evaluated first; its value must be a namespace, and the result is the value of the identifier's name in the namespace, or an error if the name is undefined. A parenthesized expression such as
"(" _modExpr ")" simply returns the result of the interior expression. A braced construct such as
BraceFunc is defined by the evaluation of the statements it contains after all parameters are accepted. Finally, a list
"⟨" ⋄? ( ( EXPR ⋄ )* EXPR ⋄? )? "⟩" or
ANY ( "‿" ANY )+ consists grammatically of a list of expressions. To evaluate it, each expression is evaluated in source order and their results are placed as elements of a rank-1 array. The two forms have identical semantics but different punctuation.
Rules in the table below are function and modifier evaluation.
In each case the constituent expressions are evaluated in reverse source order: Right, then Called, then Left. Then the expression's result is obtained by calling the Called value on its parameters. A left argument of
nothing is not used as a parameter, leaving only a right argument in that case. The type of the Called value must be appropriate to the expression type, as indicated in the "Types" column. For function application, a data type (number, character, or array) is allowed. It is called simply by returning itself. Although the arguments are ignored in this case, they are still evaluated. A braced construct is evaluated by binding the parameter names given in columns L and R to the corresponding values. Then if all parameter levels present have been bound, its body is evaluated to give the result of application.
Modifiers that are evaluated when they receive operands are called immediate. Other modifiers, including primitives and some kinds of block, simply record the operands and are called deferred. The result of applying a deferred modifier once is called a derived function.
The rules for trains create another kind of derived function. A derived function is identified by the rule that created it, and the values of its parts.
A train is a function that, when called, calls the right-hand function on all arguments, then the left-hand function, and calls the center function with these results as arguments. As with applications, all expressions are evaluated in reverse source order before doing anything else. Then a result is formed without calling the center value. Its behavior as a function is described in the rightmost column, using
R for the results of the expressions in the left, center, and right columns, respectively.