how to create a lazy tree in Standard ML

Question

I want to implement a lazy tree in Standard ML with

datatype tr = N of int * unit->tr * unit->tr;

I tried to use

fun form k = N(k,fn()=>form(k+1),fn()=>form(k+3));

as a test of concept, but I get error message saying type mismatch.

What would be the proper way of implementing a lazy tree in ML?

Answer 1

datatype tr = N of int * unit->tr * unit->tr;

This is parsed as

datatype tr = N of (int * unit) -> ( (tr * unit) -> tr)

You want

datatype tr = N of int * (unit->tr) * (unit->tr)