How to convert this map/flatMap into a for comprehension, and please explain how it works, thanks.
def compute2(maybeFoo: Option[Foo]): Option[Int] =
maybeFoo.flatMap { foo =>
foo.bar.flatMap { bar =>
bar.baz.map { baz =>
baz.compute
}
}
}
Your code can be translated into this:
def compute2(maybeFoo: Option[Foo]): Option[Int] =
for {
foo <- maybeFoo
bar <- foo.bar
baz <- bar.baz
} yield baz.compute
Quotes from Programming in Scala, Second Edition:
Generally, a for expression is of the form:
for ( seq ) yield expr
Here, seq is a sequence of generators, definitions, and filters, with semi-colons between successive elements.
This for expression contains one generator, one definition, and one filter:
for {
p <- persons // a generator
n = p.name // a definition
if (n startsWith "To") // a filter
} yield n
Translating for expressions with one generator
First, assume you have a simple for expression:
for (x <- expr1) yield expr2
where x is a variable. Such an expression is translated to:
expr1.map(x => expr2)
Translating for expressions starting with a generator and a filter
Now, consider for expressions that combine a leading generator with some
other elements. A for expression of the form:
for (x <- expr1 if expr2) yield expr3
is translated to:
expr1 withFilter (x => expr2) map (x => expr3)
Translating for expressions starting with two generators
The next case handles for expressions that start with two generators, as in:
for (x <- expr1; y <- expr2) yield expr3
The for expression above is translated to an application of flatMap:
expr1.flatMap(x => for (y <- expr2) yield expr3)