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A dialecting/DSL sample in Rebol
REBOL [
Title: "Euler1 dialecting example"
Author: "Torbjørn Marø"
Home: http://blog.kjempekjekt.com
Date: 19-Dec-2011
Purpose: {
This script demonstrates how to use the built in parse
functionality of Rebol to create an internal DSL.
This particular DSL lets you create sums that are the
result of adding og subtracting multiples of any number
below a certain limit.
}
]
fold: func [
{ Generic high-order function that folds elements of a list.
Example: fold func[acc x][acc * x] [1 2 3 4]
evaluates to 24 }
f [function!] "The folding function"
l [block!] "The elements to fold"
/local acc
] [
acc: first l ; Start with first element, loop over rest
foreach x next l [ acc: f acc x ]
]
summing-dialect: context [
{
summing-dialect is an object used purely for encapsulation
}
use [result operation multiplier below] [
{
The SUMMING word is set in the global context as a
function that takes a block of instructions as an
argument and returns the resulting sum.
}
set 'summing func [instructions] [
result: 0
parse compose instructions [any summing-rules]
return result
]
{
The parsing rules. These are the MAIN PART you have to
grok in order to use parsing and dialects in Rebol.
}
summing-rules: [
set operation [ 'Add | 'Subtract ]
'multiples
'of
set multiplier integer!
'below
set below integer!
(do-instruction) ; called when instruction has been parsed
]
{
Uses the result of parsing an instruction to create
the multiples and modify the result
}
do-instruction: has [temp] [
temp: copy [] ; block to contain the multiples
for x multiplier below - 1 multiplier [
append temp x
]
; Sum all the multiples into temp..
temp: fold func[acc x][ acc + x ] temp
; Finally, either add or subtract to/from result
result: either operation = 'Add [
result + temp
] [
result - temp
]
]
]
]
{
Then it's time to use the dialect
}
alert to-string summing []
; >> displays 0
alert to-string summing [ Add multiples of 5 below 11 ]
; >> displays 15
alert to-string summing [ Subtract multiples of 1 below 10 ]
; >> displays -45
; Display the sum of all multiples of 3 or 5 below 1000
alert to-string
summing [
Add multiples of 3 below 1000
Add multiples of 5 below 1000
; Since some numbers will have been added twice,
; we now have to subtract those numbers once..
Subtract multiples of (3 * 5) below 1000
]
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