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\DOC new_letter
\TYPE {new_letter : (string -> void)}
\SYNOPSIS
Defines a character to be a letter.
\DESCRIBE
When given a string, which should be of length 1, {new_letter} extends
the scope of the definition of `letter' as used by {is_letter}.
(If the supplied string is null ({``}) {new_letter} has no effect.)
The definition of a `letter' by default includes (only)
{a..z A..Z}. A call to {new_letter} can affect ML lexical analysis dynamically,
because the function {is_letter} is called by the lexical analyzer to check for
the start of an identifier.
\EXAMPLE
In the default state, {-1} is parsed as the application of {-} to {1}:
{
#let x = -1 ;;
x = -1 : int
}
\noindent but if we make {-} into a letter:
{
#new_letter(`-`);;
() : void
}
\noindent then {-1} parses as an (undefined) identifier.
{
#let x = -1;;
unbound or non-assignable variable -1
}
\FAILURE
Fails if the string has length greater than 1.
\SEEALSO
ascii, ascii_code, is_alphanum, is_letter, new_alphanum.
\ENDDOC
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