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diff --git a/docs/doc/expression.html b/docs/doc/expression.html index 9e386079..8f9cc5cc 100644 --- a/docs/doc/expression.html +++ b/docs/doc/expression.html @@ -62,6 +62,13 @@ <p><em>This issue is approached from a different angle in <a href="context.html">Context free grammar</a>.</em></p> <p>In APL, the way one part of an expression interacts with others is determined by its value. That means that to parse an expression, in general you would have to evaluate that part, get a value, check its type, and then figure out how it fits in with the rest of the expression. This is a lot of work. BQN changes things so that you can determine how to parse an expression just by looking at its source code. But because it still needs to support expressions that can evaluate to more than one possible <a href="types.html">type</a>, BQN has to introduce a new and independent concept, called <strong>syntactic role</strong>, in order to support APL-like expressions.</p> <p>Syntactic role is a property of an expression, not its value. To describe it in terms of English grammar, you might say "I like BQN", using "BQN" as an object, or "BQN scares me", using it as a subject. BQN itself isn't a subject or object, it's a programming language. Similarly you might write <code><span class='Function'>F</span> <span class='Value'>g</span></code>, placing <code><span class='Value'>f</span></code> in a function role to apply it to <code><span class='Value'>g</span></code>, or <code><span class='Function'>G</span> <span class='Value'>f</span></code> to use <code><span class='Value'>f</span></code> as an argument. Maybe even in the same program, although it's unlikely.</p> +<p>Below, the function <code><span class='Brace'>{</span><span class='Function'>𝕎</span><span class='Value'>𝕩</span><span class='Brace'>}</span></code> treats its left argument <code><span class='Function'>𝕎</span></code> as a function and its right argument <code><span class='Value'>𝕩</span></code> as a subject. With a list of functions, we can make a table of the square and square root of a few numbers:</p> +<a class="replLink" title="Open in the REPL" target="_blank" href="https://mlochbaum.github.io/BQN/try.html#code=4p+ow5fLnCziiJrin6kge/CdlY7wnZWpfeKMnCAx4oC/NOKAvzk=">↗️</a><pre> <span class='Bracket'>⟨</span><span class='Function'>×</span><span class='Modifier'>˜</span><span class='Separator'>,</span><span class='Function'>√</span><span class='Bracket'>⟩</span> <span class='Brace'>{</span><span class='Function'>𝕎</span><span class='Value'>𝕩</span><span class='Brace'>}</span><span class='Modifier'>⌜</span> <span class='Number'>1</span><span class='Ligature'>‿</span><span class='Number'>4</span><span class='Ligature'>‿</span><span class='Number'>9</span> +┌─ +╵ 1 16 81 + 1 2 3 + ┘ +</pre> <h3 id="role-spellings"><a class="header" href="#role-spellings">Role spellings</a></h3> <p>The four roles are <strong>subject</strong>, <strong>function</strong>, <strong>1-modifier</strong>, and <strong>2-modifier</strong>, as shown in the table below. Each type has an associated role (with non-operation types all corresponding to subjects), and the value of an expression will often have a matching type, but it doesn't have to.</p> <table> @@ -180,3 +187,26 @@ </tbody> </table> <p>A function with an asterisk indicates that a subject can also be used. Since the role doesn't exist after parsing, function and subject spellings are indistinguishable in these positions. Modifier applications bind more tightly than functions, and associate left-to-right while functions associate right-to-left.</p> +<h3 id="assignment"><a class="header" href="#assignment">Assignment</a></h3> +<p>Another element that can be included in expressions is assignment, which is written with <code><span class='Gets'>←</span></code> to <em>define</em> (also called "declare" in many other languages) a variable and <code><span class='Gets'>↩</span></code> to <em>change</em> its definition. A variable can only be defined once within a <a href="lexical.html">scope</a>, and can only be changed if it has already been defined. However, it can be shadowed, meaning that it is defined again in an inner scope even though it has a definition in an outer scope already.</p> +<a class="replLink" title="Open in the REPL" target="_blank" href="https://mlochbaum.github.io/BQN/try.html#code=eOKGkDEg4ouEIHt44oaQMiDii4QgeOKGqTMg4ouEIHh9Cng=">↗️</a><pre> <span class='Value'>x</span><span class='Gets'>←</span><span class='Number'>1</span> <span class='Separator'>⋄</span> <span class='Brace'>{</span><span class='Value'>x</span><span class='Gets'>←</span><span class='Number'>2</span> <span class='Separator'>⋄</span> <span class='Value'>x</span><span class='Gets'>↩</span><span class='Number'>3</span> <span class='Separator'>⋄</span> <span class='Value'>x</span><span class='Brace'>}</span> +3 + <span class='Value'>x</span> +1 +</pre> +<p>Assignment can be used inline in an expression, and its result is always the value being assigned. The role of the identifier used must match the value being assigned.</p> +<a class="replLink" title="Open in the REPL" target="_blank" href="https://mlochbaum.github.io/BQN/try.html#code=MsOXYeKGkChOZWfihpAtKTMKYQ==">↗️</a><pre> <span class='Number'>2</span><span class='Function'>×</span><span class='Value'>a</span><span class='Gets'>←</span><span class='Paren'>(</span><span class='Function'>Neg</span><span class='Gets'>←</span><span class='Function'>-</span><span class='Paren'>)</span><span class='Number'>3</span> +¯6 + <span class='Value'>a</span> +¯3 +</pre> +<h3 id="exports"><a class="header" href="#exports">Exports</a></h3> +<p>The double arrow <code><span class='Gets'>⇐</span></code> is used to export variables from a block or program, causing the result to be a <a href="namespace.html">namespace</a>. There are two ways to export variables. First, <code><span class='Gets'>←</span></code> in the variable definition can be replaced with <code><span class='Gets'>⇐</span></code> to export the variable as it's defined. Second, an export statement consisting of an assignment target followed by <code><span class='Gets'>⇐</span></code> with nothing to the right exports the variables in the assignment target and does nothing else. Export statements can be placed anywhere in the relevant program or body, including before declaration or on the last line, and a given variable can be exported any number of times.</p> +<pre><span class='Bracket'>⟨</span><span class='Value'>alias</span><span class='Gets'>⇐</span><span class='Value'>a</span><span class='Separator'>,</span> <span class='Value'>b</span><span class='Separator'>,</span> <span class='Value'>c0</span><span class='Ligature'>‿</span><span class='Value'>c1</span><span class='Gets'>⇐</span><span class='Value'>c</span><span class='Separator'>,</span> <span class='Value'>b2</span><span class='Gets'>⇐</span><span class='Value'>b</span><span class='Bracket'>⟩</span><span class='Gets'>←</span><span class='Brace'>{</span> + <span class='Value'>b</span><span class='Ligature'>‿</span><span class='Value'>c</span><span class='Gets'>⇐</span> <span class='Comment'># Non-definition exports can go anywhere +</span> <span class='Value'>a</span><span class='Gets'>⇐</span><span class='Number'>2</span> <span class='Comment'># Define and export +</span> <span class='Value'>b</span><span class='Gets'>←</span><span class='Number'>1</span><span class='Function'>+</span><span class='Value'>a</span> + <span class='Value'>c</span><span class='Gets'>←</span><span class='Value'>b</span><span class='Ligature'>‿</span><span class='String'>"str"</span> +<span class='Brace'>}</span> +</pre> +<p>Fields of the resulting namespace can be accessed either directly using <code><span class='Value'>namespace.field</span></code> syntax, or with a destructuring assignment as shown above. This assignment's target is a list where each element specifies one of the names exported by the block and what it should be assigned to. The element can be either a single name (such as <code><span class='Value'>b</span></code> above), which gives both, or a combination of the assignment target, then <code><span class='Gets'>⇐</span></code>, then a name. If <code><span class='Gets'>⇐</span></code> is never used, the names can be given as a strand with <code><span class='Ligature'>‿</span></code>. To use <code><span class='Gets'>⇐</span></code> for aliases, bracket syntax <code><span class='Bracket'>⟨⟩</span></code> is needed. Imported names can be repeated and can be spelled with any role (the role is ignored).</p> |