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version 2.109.1

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Template std.typecons.apply

Unpacks the content of a Nullable, performs an operation and packs it again. Does nothing if isNull.

template apply(alias fun) ;

When called on a Nullable, apply will unpack the value contained in the Nullable, pass it to the function you provide and wrap the result in another Nullable (if necessary). If the Nullable is null, apply will return null itself.

Contained Functions

NameDescription
apply

Parameters

NameDescription
t a Nullable
fun a function operating on the content of the nullable

Returns

fun(t.get).nullable if !t.isNull, else Nullable.init.

See also: The Maybe monad

Example

alias toFloat = i => cast(float) i;

Nullable!int sample;

// apply(null) results in a null `Nullable` of the function's return type.
Nullable!float f = sample.apply!toFloat;
assert(sample.isNull && f.isNull);

sample = 3;

// apply(non-null) calls the function and wraps the result in a `Nullable`.
f = sample.apply!toFloat;
assert(!sample.isNull && !f.isNull);
writeln(f.get); // 3.0f

Example

alias greaterThree = i => (i > 3) ? i.nullable : Nullable!(typeof(i)).init;

Nullable!int sample;

// when the function already returns a `Nullable`, that `Nullable` is not wrapped.
auto result = sample.apply!greaterThree;
assert(sample.isNull && result.isNull);

// The function may decide to return a null `Nullable`.
sample = 3;
result = sample.apply!greaterThree;
assert(!sample.isNull && result.isNull);

// Or it may return a value already wrapped in a `Nullable`.
sample = 4;
result = sample.apply!greaterThree;
assert(!sample.isNull && !result.isNull);
writeln(result.get); // 4

Authors

Andrei Alexandrescu, Bartosz Milewski, Don Clugston, Shin Fujishiro, Kenji Hara

License

Boost License 1.0.