Define a derived operation in the spec

1 files changed, 4 insertions, 3 deletions

diff --git a/docs/spec/evaluate.html b/docs/spec/evaluate.html
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+++ b/docs/spec/evaluate.html
@@ -11,7 +11,7 @@
 <h3 id="programs-and-blocks"><a class="header" href="#programs-and-blocks">Programs and blocks</a></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'>•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'>𝕨𝕩𝕤</span><span class='Function'>𝕎𝕏𝕊</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>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. The stored values can be accessed by equality checking, or <code><span class='Function'>•Decompose</span></code> if defined. 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'>𝕨𝕩𝕤</span><span class='Function'>𝕎𝕏𝕊</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'>𝕨</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'>𝕨𝕩𝕤</span></code> if arguments are used, <code><span class='Value'>𝕨</span></code> if there is a left argument, <code><span class='Value'>𝕗𝕘</span></code> if operands are used, and <code><span class='Modifier'>_𝕣</span></code> and <code><span class='Modifier2'>_𝕣_</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'>𝕨</span></code> or <code><span class='Function'>𝕎</span></code> at the top level, then it is conceptually re-parsed with <code><span class='Value'>𝕨</span></code> replaced by <code><span class='Nothing'>·</span></code> to give a monadic version before application; this modifies the syntax tree by replacing some instances of <code><span class='Value'>subject</span></code>, <code><span class='Value'>arg</span></code>, or <code><span class='Function'>Operand</span></code> with <code><span class='Value'>nothing</span></code>. The token <code><span class='Function'>𝕎</span></code> is not allowed in this case and causes an error. Re-parsing <code><span class='Value'>𝕨</span></code> can also cause an error if it's used as an operand or list element, where <code><span class='Value'>nothing</span></code> 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 <code><span class='Nothing'>·</span></code> 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 <code><span class='Nothing'>·</span></code> is &quot;returned&quot; immediately; and if it's used in another context then it causes an error.</p>
@@ -62,7 +62,8 @@
 </tbody>
 </table>
 <p>In each case the constituent expressions are evaluated in reverse source order: Right, then Called, then Left. Then the expression's result is obtained by calling the Called value on its parameters. A left argument of <code><span class='Value'>nothing</span></code> is not used as a parameter, leaving only a right argument in that case. The type of the Called value must be appropriate to the expression type, as indicated in the &quot;Types&quot; column. For function application, a data type (number, character, or array) is allowed. It is called simply by returning itself. Although the arguments are ignored in this case, they are still evaluated. A braced construct is evaluated by binding the parameter names given in columns L and R to the corresponding values. Then if all parameter levels present have been bound, its body is evaluated to give the result of application.</p>
-<p>The following rules derive new functions or modifiers from existing ones.</p>
+<p>Modifiers that are evaluated when they receive operands are called <em>immediate</em>. Other modifiers, including primitives and some kinds of block, simply record the operands and are called <em>deferred</em>. The result of applying a deferred modifier once is called a <em>derived function</em>.</p>
+<p>The following rules always create <em>derived operations</em>, either 1-modifiers or derived functions. A derived operation is identified by the rule that created it, and the values of its parts.</p>
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 <thead>
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@@ -99,4 +100,4 @@
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-<p>As with applications, all expressions are evaluated in reverse source order before doing anything else. Then a result is formed without calling the center value. Its value in BQN is given in the rightmost column, using <code><span class='Function'>L</span></code>, <code><span class='Function'>C</span></code>, and <code><span class='Function'>R</span></code> for the results of the expressions in the left, center, and right columns, respectively. For the first two rules (<em>partial application</em>), the given operand is bound to the 2-modifier: the result is a 1-modifier that, when called, calls the center 2-modifier with the bound operand on the same side it appeared on and the new operand on the remaining side. A <em>train</em> is a function that, when called, calls the right-hand function on all arguments, then the left-hand function, and calls the center function with these results as arguments. In a modifier partial application, the result will fail when applied if the center value does not have the 2-modifier type, and in a fork, it will fail if any component has a modifier type (that is, cannot be applied as a function). BQN implementations are not required to check for these types when forming the result of these expressions, but may give an error on formation even if the result will never be applied.</p>
+<p>As with applications, all expressions are evaluated in reverse source order before doing anything else. Then a result is formed without calling the center value. Its behavior as a function is described in the rightmost column, using <code><span class='Function'>L</span></code>, <code><span class='Function'>C</span></code>, and <code><span class='Function'>R</span></code> for the results of the expressions in the left, center, and right columns, respectively. For the first two rules (<em>partial application</em>), the given operand is bound to the 2-modifier: the result is a 1-modifier that, when called, calls the center 2-modifier with the bound operand on the same side it appeared on and the new operand on the remaining side. A <em>train</em> is a function that, when called, calls the right-hand function on all arguments, then the left-hand function, and calls the center function with these results as arguments. In a modifier partial application, the result will fail when applied if the center value does not have the 2-modifier type, and in a fork, it will fail if any component has a modifier type (that is, cannot be applied as a function). BQN implementations are not required to check for these types when forming the result of these expressions, but may give an error on formation even if the result will never be applied.</p>