How to turn this into a flatMap implementation for understanding
I tried to turn this into a flatMap / map implementation for understanding, but am struggling to figure out how:
def operationParser: Parser[(Operation, Int, Int)] = {
for {
n1 <- Parser.natural
_ <- Parser.list(Parser.space)
op <- Parser.operation
_ <- Parser.list(Parser.space)
n2 <- Parser.natural
} yield (op, n1, n2)
}
source to share
You can use the command line option -Xprint:parser
for scalac
to see the revoked version:
Source
object Main {
trait Operation
trait Parser[A] {
def map[B](fn: A => B): Parser[B] = ???
def flatMap[B](fn: A => Parser[B]): Parser[B] = ???
}
object Parser {
def natural: Parser[Int] = ???
def space: Parser[String] = ???
def list[A](p: Parser[A]): Parser[A] = ???
def operation: Parser[Operation] = ???
}
def operationParser: Parser[(Operation, Int, Int)] = {
for {
n1 <- Parser.natural
_ <- Parser.list(Parser.space)
op <- Parser.operation
_ <- Parser.list(Parser.space)
n2 <- Parser.natural
} yield (op, n1, n2)
}
}
Compiler command and output
$ scalac -Xprint:parser Main.scala
[[syntax trees at end of parser]] // Main.scala
package <empty> {
object Main extends scala.AnyRef {
def <init>() = {
super.<init>();
()
};
abstract trait Operation extends scala.AnyRef;
abstract trait Parser[A] extends scala.AnyRef {
def $init$() = {
()
};
def map[B](fn: _root_.scala.Function1[A, B]): Parser[B] = $qmark$qmark$qmark;
def flatMap[B](fn: _root_.scala.Function1[A, Parser[B]]): Parser[B] = $qmark$qmark$qmark
};
object Parser extends scala.AnyRef {
def <init>() = {
super.<init>();
()
};
def natural: Parser[Int] = $qmark$qmark$qmark;
def space: Parser[String] = $qmark$qmark$qmark;
def list[A](p: Parser[A]): Parser[A] = $qmark$qmark$qmark;
def operation: Parser[Operation] = $qmark$qmark$qmark
};
def operationParser: Parser[scala.Tuple3[Operation, Int, Int]] = Parser.natural.flatMap(((n1) => Parser.list(Parser.space).flatMap(((_) => Parser.operation.flatMap(((op) => Parser.list(Parser.space).flatMap(((_) => Parser.natural.map(((n2) => scala.Tuple3(op, n1, n2)))))))))))
}
}
After some formatting, the desugared version looks like this:
def operationParser: Parser[scala.Tuple3[Operation, Int, Int]] =
Parser.natural.flatMap(((n1) =>
Parser.list(Parser.space).flatMap(((_) =>
Parser.operation.flatMap(((op) =>
Parser.list(Parser.space).flatMap(((_) =>
Parser.natural.map(((n2) =>
scala.Tuple3(op, n1, n2)))))))))))
You can view the list of available compilation phases with this command:
$ scalac -Xshow-phases
phase name id description
---------- -- -----------
parser 1 parse source into ASTs, perform simple desugaring
namer 2 resolve names, attach symbols to named trees
packageobjects 3 load package objects
typer 4 the meat and potatoes: type the trees
patmat 5 translate match expressions
superaccessors 6 add super accessors in traits and nested classes
extmethods 7 add extension methods for inline classes
pickler 8 serialize symbol tables
refchecks 9 reference/override checking, translate nested objects
uncurry 10 uncurry, translate function values to anonymous classes
tailcalls 11 replace tail calls by jumps
specialize 12 @specialized-driven class and method specialization
explicitouter 13 this refs to outer pointers
erasure 14 erase types, add interfaces for traits
posterasure 15 clean up erased inline classes
lazyvals 16 allocate bitmaps, translate lazy vals into lazified defs
lambdalift 17 move nested functions to top level
constructors 18 move field definitions into constructors
flatten 19 eliminate inner classes
mixin 20 mixin composition
cleanup 21 platform-specific cleanups, generate reflective calls
delambdafy 22 remove lambdas
icode 23 generate portable intermediate code
jvm 24 generate JVM bytecode
terminal 25 the last phase during a compilation run
source to share
As a rule, everyone <-
becomes flatMap
except for the last one (in your case n2 < -Parser.natural
), which becomes map
. But you can "cheat" and force the compiler to do it for you by compiling with:
scalac -Xprint:parser YourClass.sca
It will show the de-sugar compiler to understand.
source to share