104 lines
2.1 KiB
Markdown
104 lines
2.1 KiB
Markdown
# Quotes and Combinators
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RETRO makes extensive use of two elements that need some explanation. These
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are called quotes and combinators.
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A *quote* is an anonymous function. They are nestable and can be created at
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any time.
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Example:
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````
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#12 [ dup * ] call
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````
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In this, the code stating with `[` and ending with `]` is the quote. Here
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it's just `call`ed immediately, but you can also pass it to other words.
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We use the word *combinator* to refer to words that operate on quotes.
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You'll use quotes and combinators extensively for controlling the flow of
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execution. This begins with conditionals.
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Assuming that we have a flag on the stack, you can run a quote if the
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flag is `TRUE`:
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````
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#1 #2 eq?
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[ 'True! puts ] if
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````
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Or if it's `FALSE`:
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````
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#1 #2 eq?
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[ 'Not_true! puts ] -if
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````
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There's also a `choose` combinator:
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````
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#1 #2 eq?
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[ 'True! puts ]
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[ 'Not_true! puts ] choose
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````
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RETRO also uses combinators for loops:
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A counted loop takes a count and a quote:
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````
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#0 #100 [ dup putn sp n:inc ] times
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````
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You can also loop while a quote returns a flag of `TRUE`:
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````
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#0 [ n:inc dup #100 lt? ] while
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````
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Or while it returns `FALSE`:
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````
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#100 [ n:dec dup n:zero? ] until
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````
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In RETRO you can also use combinators to iterate over data structures. For
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instance, many structures provide a `for-each` combinator which can be run
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once for each item in the structure. E.g., with a string:
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````
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'Hello [ putc ] s:for-each
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````
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Moving further, combinators are also used for filters and operations on
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data. Again with strings:
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````
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'Hello_World! [ c:-vowel? ] s:filter
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````
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This runs `s:filter` which takes a quote returning a flag. For each `TRUE`
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it appends the character into a new string, while `FALSE` results are
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discarded.
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You might also use a `map`ping combinator to update a data set:
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````
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'Hello_World [ c:to-upper ] s:map
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````
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This takes a quote that modifies a value which is then used to build a
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new string.
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There are many more combinators. Look in the Glossary to find them. Some
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notable ones include:
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bi
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bi*
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bi@
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tri
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tri*
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tri@
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dip
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sip
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