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xs.head :: removeDuplicates(

for (x <- xs.tail if x != xs.head) yield x

)

23.4 Translation of for expressions

Every for expression can be expressed in terms of the three higher-order functions map, flatMap, and withFilter. This section describes the translation scheme, which is also used by the Scala compiler.

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:

for (x <- expr1 withFilter (x => expr2)) yield expr3

This translation gives another for expression that is shorter by one element than the original, because an if element is transformed into an application of withFilter on the first generator expression. The translation then continues with this second expression, so in the end you obtain:

expr1 withFilter (x => expr2) map (x => expr3)

The same translation scheme also applies if there are further elements following the filter. If seq is an arbitrary sequence of generators, definitions and filters, then:

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for (x <- expr1 if expr2; seq) yield expr3

is translated to:

for (x <- expr1 withFilter expr2; seq) yield expr3

Then translation continues with the second expression, which is again shorter by one element than the original one.

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; seq) yield expr3

Again, assume that seq is an arbitrary sequence of generators, definitions and filters. In fact, seq might also be empty, and in that case there would not be a semicolon after expr2. The translation scheme stays the same in each case. The for expression above is translated to an application of flatMap:

expr1.flatMap(x => for (y <- expr2; seq) yield expr3)

This time, there is another for expression in the function value passed to flatMap. That for expression (which is again simpler by one element than the original) is in turn translated with the same rules.

The three translation schemes given so far are sufficient to translate all for expressions that contain just generators and filters, and where generators bind only simple variables. Take for instance the query, “find all authors who have published at least two books,” from Section 23.3:

for (b1 <- books; b2 <- books if b1 != b2;

a1 <- b1.authors; a2 <- b2.authors if a1 == a2) yield a1

This query translates to the following map/flatMap/filter combination:

books flatMap (b1 =>

books withFilter (b2 => b1 != b2) flatMap (b2 => b1.authors flatMap (a1 =>

b2.authors withFilter (a2 => a1 == a2) map (a2 => a1))))

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The translation scheme presented so far does not yet handle generators that bind whole patterns instead of simple variables. It also does not yet cover definitions. These two aspects will be explained in the next two sub-sections.

Translating patterns in generators

The translation scheme becomes more complicated if the left hand side of generator is a pattern, pat, other than a simple variable. Still relatively easy to handle is the case where the for expression binds a tuple of variables. In that case, almost the same scheme as for single variables applies. A for expression of the form:

for ((x1, ..., xn) <- expr1) yield expr2

translates to:

expr1.map { case (x1, ..., xn) => expr2 }

Things become a bit more involved if the left hand side of the generator is an arbitrary pattern pat instead of a single variable or a tuple. In this case:

for (pat <- expr1) yield expr2

translates to:

expr1 withFilter { case pat => true case _ => false

}map {

case pat => expr2

}

That is, the generated items are first filtered and only those that match pat are mapped. Therefore, it’s guaranteed that a pattern-matching generator will never throw a MatchError.

The scheme above only treated the case where the for expression contains a single pattern-matching generator. Analogous rules apply if the for expression contains other generators, filters, or definitions. Because these additional rules don’t add much new insight, they are omitted from discussion here. If you are interested, you can look them up in the Scala Language Specification [Ode08].

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Translating definitions

The last missing situation is where a for expression contains embedded definitions. Here’s a typical case:

for (x <- expr1; y = expr2; seq) yield expr3

Assume again that seq is a (possibly empty) sequence of generators, definitions, and filters. This expression is translated to the following one:

for ((x, y) <- for (x <- expr1) yield (x, expr2); seq) yield expr3

So you see that expr2 is evaluated each time there is a new x value being generated. This re-evaluation is necessary, because expr2 might refer to x and so needs to be re-evaluated for changing values of x. For you as a programmer the conclusion is that it’s probably not a good idea to have definitions embedded in for expressions that do not refer to variables bound by some preceding generator, because re-evaluating such expressions would be wasteful. For instance, instead of:

for (x <- 1 to 1000; y = expensiveComputationNotInvolvingX) yield x * y

it’s usually better to write:

val y = expensiveComputationNotInvolvingX for (x <- 1 to 1000) yield x * y

Translating for loops

The previous subsections showed how for expressions that contain a yield are translated. What about for loops that simply perform a side effect without returning anything? Their translation is similar, but simpler than for expressions. In principle, wherever the previous translation scheme used a map or a flatMap in the translation, the translation scheme for for loops uses just a foreach. For instance, the expression:

for (x <- expr1) body

translates to:

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