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Trampolining TCO functions in JS examples
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/* | |
* aperture function modelled after https://ramdajs.com/docs/#aperture | |
*/ | |
const head = xs => xs[0] | |
const last = xs => xs[xs.length - 1] | |
const tail = xs => xs.slice(1) | |
const isEmpty = xs => xs.length == 0 | |
/* A recursive aperture implementation | |
* | |
* | |
* Usage examples: | |
* recursive_aperture([1,2,3,4,5,6,7,8,9], 4) | |
* => [ [ 1, 2, 3, 4 ], [ 2, 3, 4, 5 ], [ 3, 4, 5, 6 ], [ 4, 5, 6, 7 ], [ 5, 6, 7, 8 ], [ 6, 7, 8, 9 ] ] | |
* recursive_aperture([1,2,3,4,5,6,7,8,9], 3) | |
* => [ [ 1, 2, 3 ], [ 2, 3, 4 ], [ 3, 4, 5 ], [ 4, 5, 6 ], [ 5, 6, 7 ], [ 6, 7, 8 ], [ 7, 8, 9 ] ] | |
* | |
*/ | |
function recursive_aperture(seq, n, acc=[]) { | |
if (isEmpty(seq)) { | |
return acc | |
} | |
if (isEmpty(acc)) { | |
return recursive_aperture(seq.slice(n), n, [seq.slice(0, n)]) | |
} | |
rem = last(acc).slice(1) | |
acc = acc.concat([rem.concat([head(seq)])]) | |
return recursive_aperture(tail(seq), n, acc) | |
} | |
/* | |
* A function that trampolines between its function argument based on returned result | |
*/ | |
function trampoline(fn) { | |
return function(...args) { | |
let nextArgs = args | |
while (true) { | |
let result = fn.apply(this, nextArgs) | |
proceed = result[0] | |
nextArgs = result[1] | |
if (!proceed) { | |
return nextArgs | |
} | |
} | |
} | |
} | |
/* | |
* The aperture function re-written to use the trampoline | |
* | |
* Examples: | |
* trampoline(aperture)([1,2,3,4,5,6,7,8,9], 2) | |
* => [ [ 1, 2 ], [ 2, 3 ], [ 3, 4 ], [ 4, 5 ], [ 5, 6 ], [ 6, 7 ], [ 7, 8 ], [ 8, 9 ] ] | |
* trampoline(aperture)([1,2,3,4,5,6,7,8,9], 4) | |
* => [ [ 1, 2, 3, 4 ], [ 2, 3, 4, 5 ], [ 3, 4, 5, 6 ], [ 4, 5, 6, 7 ], [ 5, 6, 7, 8 ], [ 6, 7, 8, 9 ] ] | |
*/ | |
function aperture(seq, n, acc=[]) { | |
if (isEmpty(seq)) { | |
return [false, acc] | |
} | |
if (isEmpty(acc)) { | |
return [true, [seq.slice(n), n, [seq.slice(0, n)]]] | |
} | |
rem = last(acc).slice(1) | |
acc = acc.concat([rem.concat([head(seq)])]) | |
return [true, [tail(seq), n, acc]] | |
} |
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