From 7c5b02227416911f6ecce3798022837539b41bfb Mon Sep 17 00:00:00 2001 From: Marshall Lochbaum Date: Wed, 2 Feb 2022 09:48:44 -0500 Subject: Grammar --- docs/spec/evaluate.html | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) (limited to 'docs/spec/evaluate.html') diff --git a/docs/spec/evaluate.html b/docs/spec/evaluate.html index 9bb02864..c9c45110 100644 --- a/docs/spec/evaluate.html +++ b/docs/spec/evaluate.html @@ -13,7 +13,7 @@

The result of parsing a valid BQN program is a PROGRAM, and the program is run by evaluating this term.

A PROGRAM or 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 β€’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 subject or 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. The 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 β€’Decompose if defined. Two calls are required if there is more than one BODY term, if the BODY contains the special names π•¨π•©π•€π•Žπ•π•Š, or if its header specifies arguments (the header-body combination is a _mCase or _cCase_). Otherwise only one is required.

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To evaluate a block when enough inputs have been received, first the correct case must be identified. To do this, first each special case (I_CASE or A_CASE), excluding A_CASE nodes whose ARG_HEAD contains "⁼", 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.

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To evaluate a block when enough inputs have been received, first the correct case must be identified. To do this, first each special case (I_CASE or A_CASE), excluding A_CASE nodes whose ARG_HEAD contains "⁼", 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 is 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 0 or 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: 𝕨𝕩𝕀 if arguments are used, 𝕨 if there is a left argument, π•—π•˜ if operands are used, and _𝕣 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 BODY contains 𝕨 or π•Ž at the top level, then it is conceptually re-parsed with 𝕨 replaced by Β· to give a monadic version before application; this modifies the syntax tree by replacing some instances of subject, arg, or Operand with nothing. The token π•Ž 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 Β· 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 Β· is "returned" immediately; and if it's used in another context then it causes an error.

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