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diff --git a/docs/spec/evaluate.html b/docs/spec/evaluate.html index 9e221c53..6fdf0a0f 100644 --- a/docs/spec/evaluate.html +++ b/docs/spec/evaluate.html @@ -7,12 +7,12 @@ <h1 id="specification-bqn-evaluation">Specification: BQN evaluation</h1> <p>This page describes the semantics of the code constructs whose grammar is given in <a href="grammar.html">grammar.md</a>. 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.</p> <p>Here we assume that the referent of each identifier, or equivalently the connections between identifiers, have been identified according to the <a href="scope.html">scoping rules</a>.</p> -<p>Errors described in this page are "evaluation errors" and can be caught by the Catch (<code><span class='Modifier2'>β</span></code>) modifier. If an error is caught, evaluation halts without attempting to complete any in-progress node, and is restarted as part of the execution of Catch.</p> +<p>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, causing an error, and returning (with <code><span class='Gets'>β</span></code>) from a block. Errors described in this page are "evaluation errors" and can be caught by the Catch (<code><span class='Modifier2'>β</span></code>) modifier. For caught errors and returns, evaluation halts without attempting to complete any in-progress node, and is restarted by Catch (for errors) or at the end of the appropriate block evaluation (for returns).</p> <h3 id="programs-and-blocks">Programs and blocks</h3> <p>The result of parsing a valid BQN program is a <code><span class='Function'>PROGRAM</span></code>, and the program is run by evaluating this term.</p> <p>A <code><span class='Function'>PROGRAM</span></code> or <code><span class='Function'>BODY</span></code> is a list of <code><span class='Function'>STMT</span></code>s, which are evaluated in program order. A result is always required for <code><span class='Function'>BODY</span></code> nodes, and sometimes for <code><span class='Function'>PROGRAM</span></code> nodes (for example, when loaded with <code><span class='Function'>β’Import</span></code>). If any identifiers in the node's scope are exported, or any of its statements is an <code><span class='Function'>EXPORT</span></code>, 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 <code><span class='Function'>STMT</span></code> node must be an <code><span class='Function'>EXPR</span></code> and its result is used. The statement <code><span class='Function'>EXPR</span></code> evaluates some APL code and possibly assigns the results, while <code><span class='Value'>nothing</span></code> evaluates any <code><span class='Value'>subject</span></code> or <code><span class='Function'>Derv</span></code> terms it contains but discards the results. An <code><span class='Function'>EXPORT</span></code> statement performs no action.</p> <p>A block consists of several <code><span class='Function'>BODY</span></code> terms, some of which may have an accompanying header describing accepted inputs and how they are processed. An immediate block <code><span class='Value'>brImm</span></code> can only have one <code><span class='Function'>BODY</span></code>, and is evaluated by evaluating the code in it. Other types of blocks do not evaluate any <code><span class='Function'>BODY</span></code> immediately, but instead return a function or modifier that obtains its result by evaluating a particular <code><span class='Function'>BODY</span></code>. The <code><span class='Function'>BODY</span></code> 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 <code><span class='Function'>BODY</span></code> term, if the <code><span class='Function'>BODY</span></code> contains the special names <code><span class='Value'>π¨π©π€</span><span class='Function'>πππ</span></code>, or if its header specifies arguments (the header-body combination is a <code><span class='Modifier'>_mCase</span></code> or <code><span class='Modifier2'>_cCase_</span></code>). Otherwise only one is required.</p> -<p>To evaluate a block when enough inputs have been received, first the correct case must be identified. To do this, first each special case (<code><span class='Function'>FCase</span></code>, <code><span class='Modifier'>_mCase</span></code>, or <code><span class='Modifier2'>_cCase_</span></code>), excluding <code><span class='Function'>FCase</span></code> nodes containing <code><span class='Function'>UndoHead</span></code>, is checked in order to see if its arguments are strucurally compatible with the given arguments. That is, is <code><span class='Value'>headW</span></code> is a <code><span class='Value'>subject</span></code>, there must be a left argument matching that structure, and if <code><span class='Value'>headX</span></code> is a <code><span class='Value'>subject</span></code>, the right argument must match that structure. This means that <code><span class='Value'>π¨</span></code> 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 (<code><span class='Function'>FMain</span></code>, <code><span class='Modifier'>_mMain</span></code>, or <code><span class='Modifier2'>_cMain_</span></code>) 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.</p> +<p>To evaluate a block when enough inputs have been received, first the correct case must be identified. To do this, first each special case (<code><span class='Function'>FCase</span></code>, <code><span class='Modifier'>_mCase</span></code>, or <code><span class='Modifier2'>_cCase_</span></code>), excluding <code><span class='Function'>FCase</span></code> nodes containing <code><span class='Function'>UndoHead</span></code>, is checked in order to see if its arguments are strucurally compatible with the given arguments. That is, is <code><span class='Value'>headW</span></code> is a <code><span class='Value'>subject</span></code>, there must be a left argument matching that structure, and if <code><span class='Value'>headX</span></code> is a <code><span class='Value'>subject</span></code>, the right argument must match that structure. This means that <code><span class='Value'>π¨</span></code> 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 (<code><span class='Function'>FMain</span></code>, <code><span class='Modifier'>_mMain</span></code>, or <code><span class='Modifier2'>_cMain_</span></code>) 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.</p> <p>The only remaining step before evaluating the <code><span class='Function'>BODY</span></code> is to bind the inputs and other names. Special names are always bound when applicable: <code><span class='Value'>π¨π©π€</span></code> if arguments are used, <code><span class='Value'>π¨</span></code> if there is a left argument, <code><span class='Value'>ππ</span></code> if operands are used, and <code><span class='Modifier'>_π£</span></code> and <code><span class='Modifier2'>_π£_</span></code> for modifiers and combinators, respectively. Any names in the header are also bound, allowing multiple assignment for arguments.</p> <p>If there is no left argument, but the <code><span class='Function'>BODY</span></code> contains <code><span class='Value'>π¨</span></code> at the top level, then it is conceptually re-parsed with <code><span class='Value'>π¨</span></code> replaced by <code><span class='Nothing'>Β·</span></code> to give a monadic version before application; this modifies the syntax tree by replacing some instances of <code><span class='Value'>arg</span></code> with <code><span class='Value'>nothing</span></code>. However, it also causes an error if, in a function that is called with no left argument, <code><span class='Value'>π¨</span></code> is used as an operand or list element, where <code><span class='Value'>nothing</span></code> is not allowed by the grammar. The same effect can also be achieved dynamically by treating <code><span class='Nothing'>Β·</span></code> 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 <code><span class='Nothing'>Β·</span></code> is "returned" immediately; and if it is used in another context then it causes an error.</p> <h3 id="assignment">Assignment</h3> @@ -21,6 +21,7 @@ <p><em>Modified assignment</em> is the subject assignment rule <code><span class='Value'>lhs</span> <span class='Function'>Derv</span> <span class='String'>"β©"</span> <span class='Value'>subExpr</span></code>. In this case, <code><span class='Value'>lhs</span></code> should be evaluated as if it were a <code><span class='Value'>subExpr</span></code> (the syntax is a subset of <code><span class='Value'>subExpr</span></code>), and the result of the function application <code><span class='Value'>lhs</span> <span class='Function'>Derv</span> <span class='Value'>subExpr</span></code> should be assigned to <code><span class='Value'>lhs</span></code>, and is also the result of the modified assignment expression.</p> <h3 id="expressions">Expressions</h3> <p>We now give rules for evaluating an <code><span class='Value'>atom</span></code>, <code><span class='Function'>Func</span></code>, <code><span class='Modifier'>_mod1</span></code> or <code><span class='Modifier2'>_mod2_</span></code> expression (the possible options for <code><span class='Function'>ANY</span></code>). A literal or primitive <code><span class='Value'>sl</span></code>, <code><span class='Function'>Fl</span></code>, <code><span class='Modifier'>_ml</span></code>, or <code><span class='Modifier2'>_cl_</span></code> has a fixed value defined by the specification (<a href="literal.html">literals</a> and <a href="primitive.html">built-ins</a>). An identifier <code><span class='Value'>s</span></code>, <code><span class='Function'>F</span></code>, <code><span class='Modifier'>_m</span></code>, or <code><span class='Modifier2'>_c_</span></code>, if not preceded by <code><span class='Value'>atom</span> <span class='String'>"."</span></code>, 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 <code><span class='Value'>atom</span> <span class='String'>"."</span></code>, then the <code><span class='Value'>atom</span></code> 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 <code><span class='String'>"("</span> <span class='Modifier'>_modExpr</span> <span class='String'>")"</span></code> simply returns the result of the interior expression. A braced construct such as <code><span class='Function'>BraceFunc</span></code> is defined by the evaluation of the statements it contains after all parameters are accepted. Finally, a list <code><span class='String'>"β¨"</span> <span class='Separator'>β</span><span class='Value'>?</span> <span class='Paren'>(</span> <span class='Paren'>(</span> <span class='Function'>EXPR</span> <span class='Separator'>β</span> <span class='Paren'>)</span><span class='Value'>*</span> <span class='Function'>EXPR</span> <span class='Separator'>β</span><span class='Value'>?</span> <span class='Paren'>)</span><span class='Value'>?</span> <span class='String'>"β©"</span></code> or <code><span class='Function'>ANY</span> <span class='Paren'>(</span> <span class='String'>"βΏ"</span> <span class='Function'>ANY</span> <span class='Paren'>)</span><span class='Function'>+</span></code> 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.</p> +<p>A <code><span class='Function'>Return</span></code> node creates a return function. As <a href="scope.html#returns">discussed</a> in the scoping rules, its identifier indicates a namespace from a particular block evaluation. When called, the function causes an error if that block has finished execution, or if the call includes a left argument <code><span class='Value'>π¨</span></code>. Otherwise, evaluation stops immediately, and resumes at the end of the block where it returns the right argument <code><span class='Value'>π©</span></code> from that block.</p> <p>Rules in the table below are function and modifier evaluation.</p> <table> <thead> |