62 lines
1.6 KiB
JavaScript
62 lines
1.6 KiB
JavaScript
// Sum-type (also called: tagged union, disjoin union, variant type)
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// A Sum-type always has only two variants, called "left" and "right".
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// Sum-types of more variants (called Enums) can be constructed by nesting Sum-types.
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import { prodType } from "./types.js";
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import { GenericType, Type } from "../primitives/types.js";
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import { typedFnType } from "./types.js";
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import { makeGeneric } from "../generics/generics.js";
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import { sumType } from "./types.js";
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export const newLeft = left => ({t: "L", v: left }); // 't': tag, 'v': value
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export const newRight = right => ({t: "R", v: right});
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// signature:
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// sum-type -> (leftType -> resultType, rightType -> resultType) -> resultType
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export const match = sum => handlers =>
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sum.t === "L"
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? handlers.l(sum.v)
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: handlers.r(sum.v);
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export const ModuleSum = {l:[
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// binary type constructor
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// Type -> Type -> Type
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...typedFnType(sumType, fnType =>
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fnType
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(Type)
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(fnType
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(Type)
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(Type)
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),
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),
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// a -> a | b
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...typedFnType(newLeft, fnType =>
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makeGeneric((a, b) =>
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fnType
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(a)
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(sumType(a)(b))
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), GenericType),
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// b -> a | b
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...typedFnType(newRight, fnType =>
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makeGeneric((a, b) =>
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fnType
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(b)
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(sumType(a)(b))
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), GenericType),
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// a | b -> (a -> c, b-> c) -> c
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...typedFnType(match, fnType =>
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makeGeneric((a, b, c) =>
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fnType
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(sumType(a)(b))
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(fnType
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(prodType
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(fnType(a)(c))
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(fnType(b)(c))
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)
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(c)
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)
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), GenericType),
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]};
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