I don't understand Factor's functors. I guess it would help to first understand what a "functor" is.
Google says:
a function; an operator.
In Factor, all functions (words) are operators, and are first-class. (In fact, I can't think of much in Factor that isn't first class). This definition isn't so helpful.
Wikipedia says:
Functor may refer to:
- ...
- In computer programming:
- function object used to pass function pointers along with its state
- ...
- In Haskell a Functor describes a generalisation of functions that perform mapping operations
The page for "function object" says:
an object to be invoked or called as if it were an ordinary function, usually with the same syntax (a function parameter that can also be a function).
So a functor is a first-class function? This is nothing special, and anyways words and quotations and stuff are already first-class in Factor.
Factor Functors have weird syntax that reminds me of generics or something.
resource:unmaintained/models/combinators/templates/templates.factor:
FROM: models.combinators => <collection> #1 ;
FUNCTOR: fmaps ( W -- )
W IS ${W}
w-n DEFINES ${W}-n
w-2 DEFINES 2${W}
w-3 DEFINES 3${W}
w-4 DEFINES 4${W}
w-n* DEFINES ${W}-n*
w-2* DEFINES 2${W}*
w-3* DEFINES 3${W}*
w-4* DEFINES 4${W}*
WHERE
MACRO: w-n ( int -- quot ) dup '[ [ _ narray <collection> ] dip [ _ firstn ] prepend W ] ;
: w-2 ( a b quot -- mapped ) 2 w-n ; inline
: w-3 ( a b c quot -- mapped ) 3 w-n ; inline
: w-4 ( a b c d quot -- mapped ) 4 w-n ; inline
MACRO: w-n* ( int -- quot ) dup '[ [ _ narray <collection> #1 ] dip [ _ firstn ] prepend W ] ;
: w-2* ( a b quot -- mapped ) 2 w-n* ; inline
: w-3* ( a b c quot -- mapped ) 3 w-n* ; inline
: w-4* ( a b c d quot -- mapped ) 4 w-n* ; inline
;FUNCTOR
The documentation is extremely sparse on these. What are they? When should I use them?
Don't think of functors as that 'They're named "functors" to annoy category theory fanboys and language purists.' :)
Their use is mainly to generate boilerplate or template code. Just like C++ templates are an optimization feature because generic dispatch can be slow, so are Factor functors.
Example here:
USING: functors io lexer namespaces ;
IN: examples.functors
FUNCTOR: define-table ( NAME -- )
name-datasource DEFINES-CLASS ${NAME}-datasource
clear-name DEFINES clear-${NAME}
init-name DEFINES init-${NAME}
WHERE
SINGLETON: name-datasource
: clear-name ( -- ) "clear table code here" print ;
: init-name ( -- ) "init table code here" print ;
name-datasource [ "hello-hello" ] initialize
;FUNCTOR
SYNTAX: SQL-TABLE: scan-token define-table ;
You can now write SQL-TABLE: person and Factor will create the words clear-person, init-person and person-datasource for you.
When to use them? I think never unless you have performance problems that warrants their use. They are very bad for grepability.