How to pattern match 0.0 in SML?

Question

I have the following code:

datatype complex = RealImg of real * real | Infinity;

fun divisionComplex(RealImg(a, b), RealImg(0.0, 0.0)) = Infinity
fun divisionComplex(RealImg(a, b), RealImg(c, d)) =
    RealImg ((a * c + b * d) / (c * c + d * d), ((b * c) - (a * d))/ (c* c + d * d))

However it fails with this:

Error: syntax error: inserting  EQUALOP

I am very confused. Why does this happen? I know that I can't compare two reals in SML, but how am I supposed to do pattern matching with 0?

This question is answered in [Why can't I compare reals in Standard ML?](https://stackoverflow.com/questions/41394992/why-cant-i-compare-reals-in-standard-ml) under **Why won't reals in patterns work [...]?** Use an epsilon. — sshine, Aug 13 '18 at 04:53

score 2 · Accepted Answer · answered Aug 12 '18 at 18:33

As you said SML doesn't allow to pattern match real numbers, but recommends to use Real.== instead or compare the difference between these number against some delta.

What about just using a mere if statement for this? (also some Infinity cases added just to make the match against function params exhaustive, but feel free to change it, because it doesn't pretend to be correct)

datatype complex = RealImg of real * real | Infinity;

fun divisionComplex(Infinity, _) = Infinity
  | divisionComplex(_, Infinity) = Infinity
  | divisionComplex(RealImg(a, b), RealImg(c, d)) =
    if Real.== (c, 0.0) andalso Real.== (d, 0.0)
    then Infinity
    else
      RealImg ((a * c + b * d) / (c * c + d * d), ((b * c) - (a * d))/ (c* c + d * d))

Thank you! Do you know anyway to match it without the `if then else` statement? — Victor Feitosa, Aug 13 '18 at 14:43
real number can't be matched in sml, since they are inexact and not an equality type — Igor Drozdov, Aug 13 '18 at 15:10

How to pattern match 0.0 in SML?

1 Answers1