Noob question: Function to prompt for input

I want to do something simple like create a function that prompts for input.
In any other language that would be trivial. I have spent 6 hours at this
today and not getting any closer. I've tried so many combinations.

Here is my latest version of the function:

fun getletter _ = (print "Please enter your guess as a character string: ";
val line = valOf (TextIO.inputLine TextIO.stdIn);
String.substring(line, 0, 4));

I get back a meaningless error message:

outer.sml:1.79 Error: syntax error: replacing SEMICOLON with RPAREN

Any ideas? Thanks in advance.
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