From 4f618598f2f31bc466343e4d304f35b53a366da6 Mon Sep 17 00:00:00 2001 From: Marshall Lochbaum Date: Mon, 15 Mar 2021 16:59:32 -0400 Subject: Finish namespace specification --- docs/spec/evaluate.html | 8 ++++---- 1 file changed, 4 insertions(+), 4 deletions(-) (limited to 'docs/spec/evaluate.html') diff --git a/docs/spec/evaluate.html b/docs/spec/evaluate.html index be087db0..d1ab7b80 100644 --- a/docs/spec/evaluate.html +++ b/docs/spec/evaluate.html @@ -9,17 +9,17 @@

Here we assume that the referent of each identifier, or equivalently the connections between identifiers, have been identified according to the scoping rules.

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 (for BODY, the last must be an EXPR, a particular kind of STMT), which are evaluated in program order. The statement EXPR evaluates some APL code and possibly assigns the results, while nothing evaluates any subject or Derv terms it contains but discards the results.

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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 APL 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 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. 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_) 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.

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.

If there is no left argument, but the BODY contains 𝕨 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 arg with nothing. However, it also causes an error if, in a function that is called with no left argument, 𝕨 is used as an operand or list element, where nothing is not allowed by the grammar. The same effect can also be achieved dynamically by treating Β· as a value and checking for it during execution. If it is used as a left argument, then the function should instead be called with no left argument (and similarly in trains); it it is 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 is used in another context then it causes an error.

Assignment

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An assignment is one of the four rules containing ASGN, other than IMPORT. 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, multiple assignment with a list left-hand side is also allowed. Multiple assignment is performed recursively by assigning right-hand-side values to the left-hand-side targets, with single-identifier (s) assignment as the base case. When matching the right-hand side to a list left-hand side, the left hand side is treated as a list of lhs targets. The evaluated right-hand side must be a list (rank-1 array) of the same length, and is matched to these targets element-wise.

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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.

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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 LHS_NAME, or an lhsList where every LHS_ENTRY is an LHS_NAME or lhs "⇐" LHS_NAME, so that it can be considered a list of LHS_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 LHS_NAME node used to get it from v.

Modified assignment is the subject assignment rule lhs Derv "↩" subExpr. In this case, lhs should be evaluated as if it were a subExpr (the syntax is a subset of subExpr), and the result of the function application lhs Derv subExpr should be assigned to lhs, and is also the result of the modified assignment expression.

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The IMPORT rule resembles a multiple assignment. However, in this case the values passed do not form a list but rather a module or namespace, which in this specification is not a value accessible to the programmer. To evaluate the IMPORT the brNS side is evaluated, then each inner variable mentioned in the nsLHS term is extracted and assigned to the corresponding outer identifier. Typically the two will both share the LHS_NAME, but if ⇐ is used in an NS_VAR then the lhs term refers to the outer identifier and LHS_NAME to the inner one. Since IMPORT is a statement and not an expression, it doesn't have a result value.

Expressions

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We now give rules for evaluating an atom, Func, _mod1 or _mod2_ expression (the possible options for ANY). A literal or primitive sl, Fl, _ml, or _cl_ has a fixed value defined by the specification (literals and built-ins). An identifier s, F, _m, or _c_ is evaluated by returning its value; because of the scoping rules it must have one when evaluated. 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 "⟨" β‹„? ( ( EXPR β‹„ )* EXPR β‹„? )? "⟩" or ANY ( "β€Ώ" 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.

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We now give rules for evaluating an atom, Func, _mod1 or _mod2_ expression (the possible options for ANY). A literal or primitive sl, Fl, _ml, or _cl_ has a fixed value defined by the specification (literals and built-ins). An identifier s, F, _m, or _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 "⟨" β‹„? ( ( EXPR β‹„ )* EXPR β‹„? )? "⟩" or ANY ( "β€Ώ" 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.

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