From 03dc07bf986051308342e7182fdb745f669f5bd2 Mon Sep 17 00:00:00 2001 From: Marshall Lochbaum Date: Sat, 27 Nov 2021 21:48:20 -0500 Subject: Specify predicates --- docs/spec/evaluate.html | 7 ++++--- docs/spec/grammar.html | 4 ++-- 2 files changed, 6 insertions(+), 5 deletions(-) (limited to 'docs') diff --git a/docs/spec/evaluate.html b/docs/spec/evaluate.html index b80c60ab..9b57dd5f 100644 --- a/docs/spec/evaluate.html +++ b/docs/spec/evaluate.html @@ -11,10 +11,11 @@

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.

Programs and blocks

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

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A PROGRAM or BODY is a list of STMTs, which are evaluated in program order. 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.

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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 the code in it. Other types of blocks do not 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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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.

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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.

To evaluate a block when enough inputs have been received, first the correct case must be identified. To do this, first each special case (FCase, _mCase, or _cCase_), excluding FCase nodes containing UndoHead, is checked in order to see if its arguments are strucurally compatible with the given arguments. That is, is headW is a subject, there must be a left argument matching that structure, and if headX is a subject, 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 (FMain, _mMain, or _cMain_) 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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The only remaining step before evaluating the BODY is to bind the inputs and other names. 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.

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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.

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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.

Assignment

An assignment is one of the four rules containing ASGN. It is evaluated by first evaluating the right-hand-side subExpr, FuncExpr, _m1Expr, or _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 lhs is lhsList or 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.

diff --git a/docs/spec/grammar.html b/docs/spec/grammar.html index 457fd039..99058319 100644 --- a/docs/spec/grammar.html +++ b/docs/spec/grammar.html @@ -84,8 +84,8 @@ | headW ( F | "π•Š" ) "˜" "⁼" headX | ( F | "π•Š" ) "˜"? "⁼" -

A braced block contains bodies, which are lists of statements, separated by semicolons and possibly preceded by headers, which are separated from the body with a colon. Multiple bodies allow different handling for various cases, which are pattern-matched by headers. For an immediate block there are no inputs, so there can only be one possible case and one body. Functions and modifiers allow any number of "matched" bodies, with headers that have arguments, followed by at most two "main" bodies with either no headers or headers without arguments. If there is one main body, it is ambivalent, but two main bodies refer to the monadic and dyadic cases.

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BODY     = PROGRAM
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A braced block contains bodies, which are lists of statements, separated by semicolons and possibly preceded by headers, which are separated from the body with a colon. A non-final expression can be made into a predicate by following it with the separator-like ?. Multiple bodies allow different handling for various cases, which are pattern-matched by headers. For an immediate block there are no inputs, so there can only be one possible case and one body. Functions and modifiers allow any number of "matched" bodies, with headers that have arguments, followed by at most two "main" bodies with either no headers or headers without arguments. If there is one main body, it is ambivalent, but two main bodies refer to the monadic and dyadic cases.

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BODY     = β‹„? ( STMT β‹„ | EXPR β‹„? "?" β‹„? )* STMT β‹„?
 FCase    = β‹„? FuncHead ":" BODY
 _mCase   = β‹„? _m1Head  ":" BODY
 _cCase_  = β‹„? _m2Head_ ":" BODY
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