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Emulating the Maybe monad in Ceylon
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/* | |
Demonstrates how to do monadic-style processing of | |
optional values in Ceylon. Represents an optional | |
type by boxing non-null values as singletons, and | |
the null value as the empty sequence. Monadic | |
operations are inherited from the stream interface, | |
with the caveat that m.flatMap(f) must be written | |
as m.flatMap(lift(f)) or as m.map(f).coalesced. | |
For the record: this is not idiomatic Ceylon. In | |
Ceylon, optional values are represented in a more | |
efficient unboxed form using a union type of form | |
T|Null. | |
*/ | |
"The type of boxed optional values." | |
alias Maybe<X> | |
given X satisfies Object | |
=> [X]|[]; | |
"A function to box an optional value. | |
(Just an alias for the built-in | |
function emptyOrSingleton().)" | |
Maybe<X> maybe<X>(X? x) | |
given X satisfies Object | |
=> emptyOrSingleton(x); | |
"A higher order function to lift a | |
function that returns an optional | |
value. The resulting function | |
returns boxed optional values." | |
Maybe<X>(Y) lift<X,Y>(X?(Y) f) | |
given X satisfies Object | |
=> compose(maybe<X>, f); | |
shared void run() { | |
value arg = maybe(process.arguments[0]); | |
//using lift() | |
arg.map(String.trimmed) | |
.flatMap(lift(parseInteger)) | |
.each(print); | |
//alternatively | |
arg.map(String.trimmed) | |
.map(parseInteger).coalesced | |
.each(print); | |
//using a comprehension | |
{ for (a in arg) | |
for (t in maybe(a.trimmed)) | |
for (p in maybe(parseInteger(t))) | |
p } | |
.each(print); | |
} |
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