In his latest Typesafe tutorial video, Alejandro Lujan explains for expressions in Scala, and provides an example of them in action. For expressions are a very useful construct that can simplify manipulation of collections and several other data structures. They can be used in place of nested for loops, or to replace calls to map and flatMap in non-collection structures. Learn more

1. Scala's
For
expressions

2. First,
an
example
• Music
label
has
ar7sts
• Ar7sts
have
albums
• Albums
have
volumes
• Volumes
have
tracks
• Tracks
have
a
name
and
dura7on

3. case class Label(artists: Seq[Artist], name: String)
case class Artist(albums: Seq[Album], name: String)
case class Album(volumes: Seq[Volume], name: String)
case class Volume(tracks: Seq[Track])
case class Track(name:String, duration: Int)

4. Get
all
track
names
from
a
volume
val volume = Volume(...)
val names: Seq[String] =
volume.tracks.map( t => t.name )

5. Get
all
track
names
from
an
album
val album = Album(...)
val names: Seq[String] =
album.volumes.flatMap(
volume => volume.tracks.map(
track => track.name
)
)

6. Get
all
track
names
from
an
ar7st
val artist = Artist(...)
val names: Seq[String] =
artist.albums.flatMap(
album => album.volumes.flatMap(
volume => volume.tracks.map(
track => track.name
)
)
)

7. Get
all
track
names
from
an
ar7st
val artist = Artist(...)
val names: Seq[String] =
for {
album <- artist.albums
volume <- album.volumes
track <- volume.tracks
} yield track.name

8. Get
all
track
names
from
a
label
val label = Label(...)
val names: Seq[String] =
for {
artist <- label.artists
album <- artist.albums
volume <- album.volumes
track <- volume.tracks
} yield track.name

9. Tracks
from
mul7-­‐volume
albums
val label = Label(...)
val names: Seq[String] =
for {
artist <- label.artists
album <- artist.albums if album.volumes.size > 1
volume <- album.volumes
track <- volume.tracks
} yield track

10. Reminders
• Any
data
structure
with
map,
flatmap
and
filter
• Seman7cs
of
these
depend
on
the
type
• For
expressions
always
return
a
value
• Type
of
resul7ng
data
structure
driven
by
the
first
generator
• Type
of
the
contained
elements
driven
by
the
yield

11. Other
examples
• Op7on
• Try
• Either
• Future