35 lines
1.4 KiB
JavaScript
35 lines
1.4 KiB
JavaScript
// m07 io: effects as values. An IO is a plain-data description of a program; nothing runs until interpreted.
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// {tag:'Pure', value} - done, with a value
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// {tag:'Effect', op, k} - perform `op`, then continue with `k(result)`
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export const pure = (value) => ({ tag: 'Pure', value })
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export const effect = (op) => ({ tag: 'Effect', op, k: pure })
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// Pure: apply f now. Effect: keep the op, and extend the continuation so f runs after k.
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// Nothing is performed here, we only build a bigger description.
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export const flatMap = (io, f) =>
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io.tag === 'Pure'
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? f(io.value)
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: { tag: 'Effect', op: io.op, k: (result) => flatMap(io.k(result), f) }
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export const map = (io, f) => flatMap(io, (a) => pure(f(a)))
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export const sequence = (ios) =>
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ios.reduceRight((rest, io) => flatMap(io, () => rest), pure(undefined))
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export const draw = (shape) => effect({ tag: 'Draw', shape })
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export const randomSeed = effect({ tag: 'RandomSeed' })
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export const drawScene = (scene) => sequence(scene.map(draw))
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// Answer each op with a canned result, collecting the ops as we go (recursion, no mutation).
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const answer = (op, seed) => (op.tag === 'RandomSeed' ? seed : undefined)
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export const trace = (io, seed) => {
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if (io.tag === 'Pure') return { result: io.value, ops: [] }
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const rest = trace(io.k(answer(io.op, seed)), seed)
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return { result: rest.result, ops: [io.op, ...rest.ops] }
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}
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