method v. fun

[David Petty]

I am new to Pyret and unclear when and why one would use methods versus functions with a data definition.

 

Here is some sample code in which the methods / functions rotr, rotl, noop all have the same semantics. Which approach is preferable and why?

# Exploration of data methods and functions.

# data type defining a triple of values and its methods
data Triple:
  | no-triple with:
    method rotr(self): no-triple end,
    method rotl(self): no-triple end,
    method noop(self): no-triple end,
  | triple(a, b, c) with:
    method rotr(self): triple(self.c, self.a, self.b) end,
    method rotl(self): triple(self.b, self.c, self.a) end,
    method noop(self): triple(self.a, self.b, self.c) end,
where:
  no-triple satisfies is-Triple
  no-triple satisfies is-no-triple
  triple('1', '2', '3') satisfies is-Triple
  triple('1', '2', '3') satisfies is-triple
  no-triple.rotr() is no-triple
  no-triple.rotl() is no-triple
  no-triple.noop() is no-triple
  triple('1', '2', '3').rotr() is triple('3', '1', '2')
  triple('1', '2', '3').rotl() is triple('2', '3', '1')
  triple('1', '2', '3').noop() is triple('1', '2', '3')
end

fun rotr(trip):
  doc: 'return trip rotated right'
  cases (Triple) trip:
    | no-triple => no-triple
    | triple(x, y, z) => triple(z, x, y)
  end
where:
  rotr(no-triple) is no-triple
  rotr(triple(1, 2, 3)) is triple(3, 1, 2)
end

fun rotl(trip):
  doc: 'return trip rotated left'
  cases (Triple) trip:
    | no-triple => no-triple
    | triple(x, y, z) => triple(y, z, x)
  end
where:
  rotl(no-triple) is no-triple
  rotl(triple(1, 2, 3)) is triple(2, 3, 1)
end

fun noop(trip):
  doc: 'return trip unchanged'
  cases (Triple) trip:
    | no-triple => no-triple
    | triple(x, y, z) => triple(x, y, z)
  end
where:
  noop(no-triple) is no-triple
  noop(triple(1, 2, 3)) is triple(1, 2, 3)
end

Thanks. 

- dcp