CS 334: CS334: ML Hints

Quitting The Interpreter

Type Ctrl-D to exit the ML interpreter if you run it interactively.

Source Code Comments

Comments in ML have the form (* ... *).

Output Messages Containing "..." or "#"

By default, ML abbreviates large lists, data structures, and strings when printing them. For example, it may print \"[1,2,3,4,....]\" instead of the complete contents of a list, or \"Node(Node(Node(#,#),#),Leaf(3))\" where the #'s represent unprinted parts of a datatype value. Include the following line at the top of the file to print larger datatype values before resorting to #.

Control.Print.printDepth := 100;

Similarly, include the following lines to print long lists and strings:

Control.Print.printLength := 500;
Control.Print.stringDepth := 500;

Warning: calling polyEqual

ML reports a warning whenever you use = to compare two values with polymorphic type. For example,

fun eq(x,y) = (x = y);

will cause this warning to be generated, because x and y will have polymorphic type ''a. This is perfectly fine and you may ignore the warning. It is not reporting any kind of semantic error or type error in your code.

The compiler reports the warning because testing whether two values are equal. For example, imagine that x and y are lists. It could take a looong time to check if they are equal if they contain many elements.

Error: duplicate variable in pattern(s)

You may only use a name once in any pattern. Thus,

fun eq(x,x) = true
  | eq(x,y) = false;

will give you an error. You would need to rewrite the above as we wrote it in the last section:

fun eq(x,y) = (x = y);

Warning: match nonexhaustive

You are performing pattern matching, but not handling all cases. In other words, some value exists that does not match any of the patterns you provide. If pattern matching is applied to such a value, the program will fail at run time with an exception.

If you are pattern matching on lists, be sure to handle the nil and non-empty cases. For datatypes, be sure to handle each different constrctor form with its own case.

Other Common Errors

If you are getting syntax errors that you cannot decipher, double check the following:

• Be sure to have ; in the right places --- it is a separator, not a terminator:

  sml fun f(x) = let val z = 3; val y = 2 in print z; print y end;

• Be sure that datatype constructors occuring in patterns are surrounded by parenthesis:

  sml fun maptree (LEAF(x)) = ... (* OK *) | ...

  instead of

  sml fun maptree LEAF(x) = ... (* BAD *) | ...

  If all else fails, you may find it useful to consult the complete list of SML compiler messages and what they mean.