vigoo / simpp

A simple pretty printer library



Build Status codecov Apache 2 License License

A simple pretty printer library primarily targeting source code generation.

Built on top of eff.


Add the following dependency:

libraryDependencies += "io.github.vigoo" %% "simpp" % "0.2.0"

Use the PrettyPrint[AdditionalFx] trait as a context to define the pretty printer. This defines the pretty printer context PrettyPrinterContext which is a combination of a writer and state effect plus the given effects specified in the AdditionalFx parameter. The pretty printer functions must have type PP[A] which is in fact Eff[PrettyPrinterContext[AdditionalFx], A].

You have to define the runAdditionalFx method to specify how the custom effects are to be interpreted.

In the simple case there are no additional effects:

import io.github.vigoo.simpp._
import org.atnos.eff.{Eff, NoFx}

object SamplePrettyPrint extends PrettyPrint[NoFx] {
  override def runAdditionalFx(f: PP[Unit]): Eff[PrettyPrinterContext[NoFx], Unit] = f
  // Define pretty printing functions here
  // Define implicit pretty printers here 

Then you will have a SamplePrettyPrint.print method for any value of type T with an implicit PrettyPrinter[T, AdditionalFx] instance.

The idea is to:

  • Specify pretty printing functions of type PP[A]. There is a couple of basic ones available in the PrettyPrint trait.
  • Specify implicit instances of the PrettyPrinter type class
  • Use SamplePrettyPrint.print to render values to strings

Pretty printing functions

Pretty printing functions can be composed from more atomic pretty printing functions. The basic ones are the following:

  • empty: PP[Unit]
  • append(value: String): PP[Unit]
  • indent(): PP[Unit]
  • unindent(): PP[Unit]
  • pretty[I](value: I)(implicit prettyPrinter: PrettyPrinter[I])

There are additional functions to wrap values, render sequences, indent blocks, etc.

Pretty printer instances

The PrettyPrinter type class provides a single method prettyPrint that can be defined by composing the above defined pretty printer functions.

For example:

implicit val expressionPrettyPrinter: PrettyPrinter[BcExpression, NoFx] = {
  case BcExpressions.Number(value) => code(value.toString)
  case BcExpressions.Add(x, y) => parenthesed(x) >> space >> code("+") >> space >> parenthesed(y)
  case BcExpressions.Sub(x, y) => parenthesed(x) >> space >> code("-") >> space >> parenthesed(y)
  case BcExpressions.Mul(x, y) => parenthesed(x) >> space >> code("*") >> space >> parenthesed(y)
  case BcExpressions.Div(x, y) => parenthesed(x) >> space >> code("/") >> space >> parenthesed(y)

Additional effects

Additional effects such as custom state can be applied to the pretty printers AdditionalFx type parameter.

For example the code below keeps track whether we are inside a string or not:

case class BashPrettyPrinterState(inString: Boolean)

object BashPrettyPrint extends PrettyPrint[Fx.fx1[State[BashPrettyPrinterState, ?]]] {
  override def runAdditionalFx(f: PP[Unit]): Eff[PrettyPrinterContext[NoFx], Unit] =
    f.evalState(BashPrettyPrinterState(inString = false))

  type BashFx = Fx.fx1[State[BashPrettyPrinterState, ?]]
  def getBashState: Eff[R, BashPrettyPrinterState] =
    get[R, BashPrettyPrinterState]

  def setBashState(newState: BashPrettyPrinterState): Eff[R, Unit] =
    put[R, BashPrettyPrinterState](newState)

  def inString[A](inner: Eff[R, A]): Eff[R, A] =
    for {
      state <- getBashState
      _ <- setBashState(state.copy(inString = true))
      result <- inner
      _ <- setBashState(state.copy(inString = state.inString))
    } yield result

  // ...