Error SML: unbound variable or constructor

Question

I am new to SML. I tried to create and test the following function below, but I received an error. I do not know what is the problem.

fun isOld(pFirstTuple: int*int*int, pSecondTuple: int*int*int) = 
    if (#1 pFirstTuple) < (#1 pSecondTuple)
    then 
        true
    if (#1 pFirstTuple) = (#1 pSecondTuple)
    then 
        if (#2 pFirstTuple) < (#2 pSecondTuple)
        then
            true

    else
        false

I have tried this command "val p = isOld((8,9,10),(10,11,12))", but it showed me the following error Unbound variable or constructor. How do I fix this?

Sam Westrick · Accepted Answer · 2018-03-03 18:17:16Z

Here's what your code looks like, stripped down by ignoring various subexpressions (replacing them with A, B, and C)

if A
then true
if B
then if C
     then true
else false

You're making extensive use of if/then/else, but the syntax is not quite correct. In SML, every if must have both a then and else clause associated with it. Here's my guess at what you actually meant:

if A
then true
else if B
then if C
     then true
     else false
else false

This is starting to get quite messy---but you can clean it up with boolean logic. Notice, for example, that if X then true else false means exactly the same thing as simply writing X, because both expressions are type bool and will always evaluate to the same boolean, regardless of what X is. You can extend this reasoning to see that

if X then true else Y is equivalent to X orelse Y.
if X then Y else false is equivalent to X andalso Y.

With these ideas, we can clean up your code considerably:

A orelse (B andalso C)

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