Here is a package that converts strings to numbers. It is only to be considered as a starting point. There is no error checking, it is probably slow and certainly buggy...

)abbrev package STRCNV StringConversions
StringConversions(): Exports == Implementation where
    Exports == with

      coerce: String -> Integer

      coerce: String -> Fraction Integer

      coerce: String -> List Fraction Integer

      coerce: String -> List Integer

      coerce: String -> Set Integer

    Implementation == add

sexfloat:SExpression:=convert(coerce("Float")@Symbol)$SExpression

      coerce(s: String): Fraction Integer ==
        if not NUMBERP(READ_-FROM_-STRING(s)$Lisp)$Lisp
        then error "coerce: String -> Fraction Integer: not a number"
        else
          sex := interpret(packageTran(ncParseFromString(s)$Lisp)$Lisp)$Lisp
          if (car car sex = sexfloat) then
             retract((cdr cdr sex) pretend Float)@Fraction(Integer)
          else
            if integer?(cdr sex) then
              ((cdr sex) pretend Integer)::Fraction Integer
            else
              (cdr cdr sex) pretend Fraction Integer

      coerce(s: String): Integer ==
        PARSE_-INTEGER(s)$Lisp

      coerce(r: String): List Fraction Integer ==
        map(coerce #1, split(r, char(" "))$String)_
          $FiniteLinearAggregateFunctions2(String, List String, Fraction Integer, _
                                           List Fraction Integer)

      coerce(r: String): List Integer ==
        map(coerce(#1), split(r, char(" "))$String)_
          $FiniteLinearAggregateFunctions2(String, List String, Integer, _
                                           List Integer)

      coerce(r: String): Set Integer ==
        s: String := delete(delete(r, #r), 1)
        brace(map(PARSE_-INTEGER(#1)$Lisp, split(s, char("
"))$String)_
          $FiniteLinearAggregateFunctions2(String, List String, Integer, _
                                           List Integer))$Set(Integer)

Compiling FriCAS source code from file 
      /var/zope2/var/LatexWiki/1862277760152673546-25px.001.spad using 
      old system compiler.
   STRCNV abbreviates package StringConversions 
------------------------------------------------------------------------
   initializing NRLIB STRCNV for StringConversions 
   compiling into NRLIB STRCNV 
   compiling exported coerce : String -> Fraction Integer
Time: 0.10 SEC.

   compiling exported coerce : String -> Integer
      STRCNV;coerce;SI;2 is replaced by PARSE-INTEGER 
Time: 0 SEC.

   compiling exported coerce : String -> List Fraction Integer
Time: 0.11 SEC.

   compiling exported coerce : String -> List Integer
Time: 0.03 SEC.

   compiling exported coerce : String -> Set Integer
Time: 0.04 SEC.

(time taken in buildFunctor:  0)

;;;     ***       |StringConversions| REDEFINED

;;;     ***       |StringConversions| REDEFINED
Time: 0 SEC.

   Cumulative Statistics for Constructor StringConversions
      Time: 0.28 seconds

   finalizing NRLIB STRCNV 
   Processing StringConversions for Browser database:
--->-->StringConversions((coerce ((Integer) (String)))): Not documented!!!!
--->-->StringConversions((coerce ((Fraction (Integer)) (String)))): Not documented!!!!
--->-->StringConversions((coerce ((List (Fraction (Integer))) (String)))): Not documented!!!!
--->-->StringConversions((coerce ((List (Integer)) (String)))): Not documented!!!!
--->-->StringConversions((coerce ((Set (Integer)) (String)))): Not documented!!!!
--->-->StringConversions(constructor): Not documented!!!!
--->-->StringConversions(): Missing Description
------------------------------------------------------------------------
   StringConversions is now explicitly exposed in frame initial 
   StringConversions will be automatically loaded when needed from 
      /var/zope2/var/LatexWiki/STRCNV.NRLIB/code

Let's try it:

coerce("123.12")@FRAC INT

(1)