Reading with interpretation (`readvals' and `$+')

There is a function readvals which takes a string  representing  a  UNIX
pathname,  and  returns a list of values found in the file of that name.
The values may be represented by arbitrary Miranda expressions,  written
one  per  line.   Blank  lines,  and  Miranda style comments (||...) are
ignored.  If the input file appears to be a teletype, readvals reacts to
syntactically  incorrect  or wrongly typed data by prompting the user to
repeat the line, and such bad values are omitted from the  result  list.
If  the  input  file  does  not appear to be a teletype, bad data causes
readvals to abort with an error message.

Note that, similarly to show
 (i) readvals is a reserved word, not an identifier.
 (ii) the context in which it is used must be such as to  determine  its
type  monomorphically.   Extra  type specifications may be needed in the
script to meet this condition.

Here is a simple example of how readvals might be used in a script
	x :: [num]
	x = readvals "data"
The file `data' should contain expressions of type num (one per line).

The readvals function provides  Miranda  with  a  simple  form  of  data
persistence - data can be written to a file (e.g. using `show') and read
back using readvals in a later session.  Data objects saved in this  way
must  of  course  be  finite.  Notice also that if you wish to save data
containing functions, you will have to set up some special  arrangement,
since such data cannot be written out using `show'.

Data of abstract type can be written to file using show  and  read  back
with  readvals - provided that an appropriate show-function was included
in the signature of the abstract type (see manual  section  on  abstract
types).

Finally note that $+ behaves exactly like an application of readvals  to
the  name  of  the  file  to which the standard input is connected.  For
example
	sum $+
read a sequence of numeric expressions from the keyboard (one per  line)
up to the next control-D, and then returns their sum.

