62 lines
1.5 KiB
JavaScript
62 lines
1.5 KiB
JavaScript
/*! Maybe
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!*/
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/*[Definition]*/
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/** data Maybe a = Just a | Nothing
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The Maybe datatype. Useful when a computation may or may not
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result in a value (e.g. logarithm is defined only for positive numbers).
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**/
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/*[Basic Utilities]*/
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/** maybe : b -> (a -> b) -> Maybe a -> b
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Apply a function to the contents of a `Maybe`.
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Return default when given `Nothing`.
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**/
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/** isJust : Maybe a -> Bool
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Check if constructed with `Just`.
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**/
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/** isNothing : Maybe a -> Bool
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Check if constructed with `Nothing`.
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**/
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/*[Maybe with Lists]*/
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/** cons : Maybe a -> [a] -> [a]
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If `Just`, adds the value to the front of the list.
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If `Nothing`, list is unchanged.
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**/
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/** justs : [Maybe a] -> [a]
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Filters out Nothings and extracts the remaining values.
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**/
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Elm.Maybe = function() {
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function consMaybe(x) { return function(xs) {
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if (x[0] === "Just") return ["Cons", x[1], xs];
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return xs;
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};
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}
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function mapCons(f) { return function(y) { return function(xs) {
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var x = f(y);
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if (x[0] === "Just") return ["Cons", x[1], xs];
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return xs;
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};
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};
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}
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function maybe(b) { return function(f) { return function(m) {
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if (m[0] === "Just") return f(m[1]);
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return b;
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};
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};
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}
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return {Just : function(x) { return ["Just",x]; },
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Nothing : ["Nothing"],
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justs : Elm.List.foldr(consMaybe)(["Nil"]),
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isJust : function(m) { return m[0] === "Just"; },
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isNothing : function(m) { return m[0] === "Nothing"; },
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cons : consMaybe,
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maybe : maybe
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};
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}();
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