It appears that the functions AR.isOk and AR.isError are defined in the typescript definitions here:

however they are not implemented in the underling Rescript.

this means that calling one of these functions results in the following:

TypeError: import_ts_belt.AR.isOk is not a function

and logging out these values:

console.log(AR.isOk);
console.log(AR.isError);

results in:

undefined
undefined