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fields on types with multiple cases
f#compiler

Hi everyone,

I'm writing a tiny compiler for the language shown below:

type pos = int * int

type expr =

Var of pos * string

| IntLit of pos * int

| Plus of pos * expr * expr

pos represents the source location (line, column) of the corresponding expression. Is it possible to make pos into a field of the type expr instead of having to repeat it in every case?

For example, in Scala I can write the following:

trait Expr {

var pos: (Int, Int);

}

case class Var(x: String) extends Expr

case class IntLit(x: Int) extends Expr

case class Plus(e1: Expr, e2: Expr) extends Expr

Is it possible to write something similar in F#?

Thanks in advance for any help!

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As you have no traits, an alternative is write your sum-type in this way:

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type pexpr = pos * expr
and expr =
   | Var of pos * string
  

| IntLit of pos * int
  

| Plus of pos * pexpr * pexpr

You may want to define also some active-patterns to simplify your code.

Laurent.

By on 10/6/2009 9:11 AM ()Reply
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By the way, there was a similar discussion here: [link:cs.hubfs.net]

Laurent.

By on 10/6/2009 9:14 AM ()Reply
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Thanks! Those answers were both very helpful. :-)

By on 10/7/2009 1:08 AM ()Reply
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F# doesn't support traits. ([link:cs.hubfs.net])

But, you could probably use interfaces and objects to do what you need (like you do in the scala example) rather than types, although it looks to me like the ML style types are cleaner.

See here for the syntax on objects ([link:msdn.microsoft.com])

By on 10/6/2009 7:43 AM ()Reply

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