From 23941024161ee15edb19f809f5cc29d53fe8fd25 Mon Sep 17 00:00:00 2001 From: Marshall Lochbaum Date: Tue, 22 Sep 2020 13:06:57 -0400 Subject: Change spec so function and left arg are still evaluated if right arg is nothing --- docs/spec/evaluate.html | 4 ++-- 1 file changed, 2 insertions(+), 2 deletions(-) (limited to 'docs/spec') diff --git a/docs/spec/evaluate.html b/docs/spec/evaluate.html index 1df68613..bfe9cbef 100644 --- a/docs/spec/evaluate.html +++ b/docs/spec/evaluate.html @@ -9,11 +9,11 @@

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 nothing does nothing when evaluated, while EXPR evaluates some APL code and possibly assigns the results, as described below.

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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.

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 ๐•จ๐•ฉ๐•ค๐•Ž๐•๐•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 ๐•จ 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: ๐•จ๐•ฉ๐•ค if arguments are used, ๐•จ if there is a left argument, ๐•—๐•˜ if operands are used, and _๐•ฃ and _๐•ฃ_ for modifiers and combinators, respectively. Any names in the header are also bound, allowing multiple assignment for arguments.

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If there is no left argument, but the BODY contains ๐•จ at the top level, then it is conceptually re-parsed with ๐•จ replaced by ยท to give a monadic version before application. As the only effect when this re-parsed form is valid is to change some instances of arg to nothing, this can be achieved efficiently by annotating parts of the AST that depend on ๐•จ as conditionally-nothing. However, it also causes an error if ๐•จ is used as an operand or list element, where nothing is not allowed by the grammar.

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If there is no left argument, but the BODY contains ๐•จ at the top level, then it is conceptually re-parsed with ๐•จ replaced by ยท 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, ๐•จ 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 ยท 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 ยท is "returned" immediately; and if it is 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, 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.

Modified assignment is the subject assignment rule lhs Derv "โ†ฉ" 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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