239 lines
14 KiB
HTML
239 lines
14 KiB
HTML
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<p><span class="h1">Using Combinators</span>
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<br/><br/>
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A combinator is a function that consumes functions as input.
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They are used heavily by the RETRO system.
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<br/><br/>
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<span class="h2">Types of Combinators</span>
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<br/><br/>
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Combinators are divided into three primary types: compositional,
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execution flow, and data flow.
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<br/><br/>
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<span class="h2">Compositional</span>
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<br/><br/>
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A compositional combinator takes elements from the stack and
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returns a new quote.
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<br/><br/>
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<span class="tt">curry</span> takes a value and a quote and returns a new quote
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applying the specified quote to the specified value. As an
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example,
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='colon'>:acc</span> <span class='note'>(n-)</span> here <span class='prim'>swap</span> , <span class='imm'>[</span> <span class='prim'>dup</span> v:inc <span class='prim'>fetch</span> <span class='imm'>]</span> curry <span class='imm'>;</span> </span><br/>
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<span class="tt">```</span></span><br/><br/>
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This would create an accumulator function, which takes an
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initial value and returns a quote that will increase the
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accumulator by 1 each time it is invoked. It will also return
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the latest value. So:
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#10</span> acc </span><br/>
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<span class="tt"><span class='prim'>dup</span> <span class='prim'>call</span> n:put </span><br/>
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<span class="tt"><span class='prim'>dup</span> <span class='prim'>call</span> n:put </span><br/>
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<span class="tt"><span class='prim'>dup</span> <span class='prim'>call</span> n:put </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="h2">Execution Flow</span>
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<br/><br/>
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Combinators of this type execute other functions.
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<br/><br/>
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<span class="h3">Fundamental</span>
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<br/><br/>
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<span class="tt">call</span> takes a quote and executes it immediately.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='imm'>[</span> <span class='num'>#1</span> n:put <span class='imm'>]</span> <span class='prim'>call</span> </span><br/>
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<span class="tt"><span class='ptr'>&words</span> <span class='prim'>call</span> </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="h3">Conditionals</span>
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<br/><br/>
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RETRO provides three primary combinators for use with
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conditional execution of quotes. These are <span class="tt">choose</span>, <span class="tt">if</span>,
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and <span class="tt">-if</span>.
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<br/><br/>
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<span class="tt">choose</span> takes a flag and two quotes from the stack. If the
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flag is true, the first quote is executed. If false, the
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second quote is executed.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#-1</span> <span class='imm'>[</span> <span class='str'>'true</span> s:put <span class='imm'>]</span> <span class='imm'>[</span> <span class='str'>'false</span> s:put <span class='imm'>]</span> choose </span><br/>
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<span class="tt"> <span class='num'>#0</span> <span class='imm'>[</span> <span class='str'>'true</span> s:put <span class='imm'>]</span> <span class='imm'>[</span> <span class='str'>'false</span> s:put <span class='imm'>]</span> choose </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="tt">if</span> takes a flag and one quote from the stack. If the flag is
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true, the quote is executed. If false, the quote is discarded.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#-1</span> <span class='imm'>[</span> <span class='str'>'true</span> s:put <span class='imm'>]</span> <span class='prim'>if</span> </span><br/>
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<span class="tt"> <span class='num'>#0</span> <span class='imm'>[</span> <span class='str'>'true</span> s:put <span class='imm'>]</span> <span class='prim'>if</span> </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="tt">-if</span> takes a flag and one quote from the stack. If the flag is
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false, the quote is executed. If true, the quote is discarded.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#-1</span> <span class='imm'>[</span> <span class='str'>'false</span> s:put <span class='imm'>]</span> -if </span><br/>
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<span class="tt"> <span class='num'>#0</span> <span class='imm'>[</span> <span class='str'>'false</span> s:put <span class='imm'>]</span> -if </span><br/>
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<span class="tt">```</span></span><br/><br/>
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RETRO also provides <span class="tt">case</span> and <span class="tt">s:case</span> for use when you have
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multiple values to check against. This is similar to a <span class="tt">switch</span>
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in C.
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<br/><br/>
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<span class="tt">case</span> takes two numbers and a quote. The initial value is
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compared to the second one. If they match, the quote is
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executed. If false, the quote is discarded and the initial
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value is left on the stack.
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<br/><br/>
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Additionally, if the first value was matched, <span class="tt">case</span> will exit
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the calling function, but if false, it returns to the calling
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function.
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<br/><br/>
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<span class="tt">s:case</span> works the same way, but for strings instead of simple
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values.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='colon'>:test</span> <span class='note'>(n-)</span> </span><br/>
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<span class="tt"> <span class='num'>#1</span> <span class='imm'>[</span> <span class='str'>'Yes</span> s:put <span class='imm'>]</span> case </span><br/>
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<span class="tt"> <span class='num'>#2</span> <span class='imm'>[</span> <span class='str'>'No</span> s:put <span class='imm'>]</span> case </span><br/>
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<span class="tt"> <span class='prim'>drop</span> <span class='str'>'No</span> idea s:put <span class='imm'>;</span> </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="h3">Looping</span>
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<br/><br/>
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Several combinators are available for handling various looping
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constructs.
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<br/><br/>
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<span class="tt">while</span> takes a quote from the stack and executes it repeatedly
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as long as the quote returns a true flag on the stack. This flag
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must be well formed and equal -1 or 0.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#10</span> <span class='imm'>[</span> <span class='prim'>dup</span> n:put sp n:dec <span class='prim'>dup</span> 0 <span class='prim'>-eq?</span> <span class='imm'>]</span> while </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="tt">times</span> takes a count and quote from the stack. The quote will
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be executed the number of times specified. No indexes are pushed
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to the stack.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#1</span> <span class='num'>#10</span> <span class='imm'>[</span> <span class='prim'>dup</span> n:put sp n:inc <span class='imm'>]</span> times <span class='prim'>drop</span> </span><br/>
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<span class="tt">```</span></span><br/><br/>
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There is also a <span class="tt">indexed-times</span> variation that provides
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access to the loop index (via <span class="tt">I</span>) and parent loop indexes
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(via <span class="tt">J</span> and <span class="tt">K</span>).
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#10</span> <span class='imm'>[</span> I n:put sp <span class='imm'>]</span> indexed-times </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="h2">Data Flow</span>
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<br/><br/>
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These combinators exist to simplify stack usage in various
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circumstances.
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<br/><br/>
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<span class="h3">Preserving</span>
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<br/><br/>
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Preserving combinators execute code while preserving portions
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of the data stack.
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<br/><br/>
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<span class="tt">dip</span> takes a value and a quote, moves the value off the main
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stack temporarily, executes the quote, and then restores the
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value.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#10</span> <span class='num'>#20</span> <span class='imm'>[</span> n:inc <span class='imm'>]</span> dip </span><br/>
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<span class="tt">```</span></span><br/><br/>
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Would yield the following on the stack:
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt">11 20 </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="tt">sip</span> is similar to <span class="tt">dip</span>, but leaves a copy of the original
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value on the stack during execution of the quote. So:
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#10</span> <span class='imm'>[</span> n:inc <span class='imm'>]</span> sip </span><br/>
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<span class="tt">```</span></span><br/><br/>
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Leaves us with:
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt">11 10 </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="h3">Cleave</span>
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<br/><br/>
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Cleave combinators apply multiple quotations to a single value
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or set of values.
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<br/><br/>
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<span class="tt">bi</span> takes a value and two quotes, it then applies each quote to
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a copy of the value.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#100</span> <span class='imm'>[</span> n:inc <span class='imm'>]</span> <span class='imm'>[</span> n:dec <span class='imm'>]</span> bi </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="tt">tri</span> takes a value and three quotes. It then applies each quote
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to a copy of the value.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#100</span> <span class='imm'>[</span> n:inc <span class='imm'>]</span> <span class='imm'>[</span> n:dec <span class='imm'>]</span> <span class='imm'>[</span> <span class='prim'>dup</span> <span class='prim'>*</span> <span class='imm'>]</span> tri </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="h3">Spread</span>
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<br/><br/>
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Spread combinators apply multiple quotations to multiple values.
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The asterisk suffixed to these function names signifies that
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they are spread combinators.
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<br/><br/>
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<span class="tt">bi*</span> takes two values and two quotes. It applies the first
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quote to the first value and the second quote to the second
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value.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#1</span> <span class='num'>#2</span> <span class='imm'>[</span> n:inc <span class='imm'>]</span> <span class='imm'>[</span> <span class='num'>#2</span> <span class='prim'>*</span> <span class='imm'>]</span> bi* </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="tt">tri*</span> takes three values and three quotes, applying the
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first quote to the first value, the second quote to the
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second value, and the third quote to the third value.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#1</span> <span class='num'>#2</span> <span class='num'>#3</span> <span class='imm'>[</span> n:inc <span class='imm'>]</span> <span class='imm'>[</span> <span class='num'>#2</span> <span class='prim'>*</span> <span class='imm'>]</span> <span class='imm'>[</span> n:dec <span class='imm'>]</span> tri* </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="h3">Apply</span>
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<br/><br/>
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Apply combinators apply a single quotation to multiple values.
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The @ sign suffixed to these function names signifies that they
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are apply combinators.
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<br/><br/>
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<span class="tt">bi@</span> takes two values and a quote. It then applies the quote to
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each value.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#1</span> <span class='num'>#2</span> <span class='imm'>[</span> n:inc <span class='imm'>]</span> bi@ </span><br/>
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<span class="tt">```</span></span><br/><br/>
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<span class="tt">tri@</span> takes three values and a quote. It then applies the quote
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to each value.
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<br/><br/>
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<span class='codeblock'><span class="tt">```</span><br/><span class="tt"><span class='num'>#1</span> <span class='num'>#2</span> <span class='num'>#3</span> <span class='imm'>[</span> n:inc <span class='imm'>]</span> tri@ </span><br/>
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<span class="tt">```</span></span><br/><br/>
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RETRO also provides <span class="tt">for-each</span> combinators for various data
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structures. The exact usage of these varies; consult the
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Glossary and relevant chapters for more details on these.
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</p>
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</body></html>
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