Exercises: A naive CSV parser

Assuming the following simplified CSV format:

• rows are separated by newlines b"\n"
• individual columns are separated by b","
• there are not separators anywhere else (e.g., no , in quoted column values)

A sample input would be

I,a,ABC
J,b,DEF
K,c,GHI

For testing you can use the -f flag to spicy-dump or spicy-driver to read input from a file instead of stdin, e.g.,

spicy-driver csv_naive.spicy -f input.csv

1. Write a parser which extracts the bytes on each row into a vector.

   Hint 1

   You top-level parser should contain a vector of rows which has unspecified length.

   Hint 2

   Define a new parser for a row which parses bytes until it finds a newline and consumes it.

   Solution

   module csv_naive;
   
   public type CSV = unit {
       rows: Row[];
   };
   
   type Row = unit {
       data: bytes &until=b"\n";
   };
   

2. Extend your parser so it also extracts individual columns (as bytes) from each row.

   Hint

   The &convert attribute allows changing the value and/or type of a field after it has been extracted. This allows you to split the row data into columns.

   Is there a builtin function which splits your row data at a separator (consuming the iterator)? Functions on bytes are documented here. You can access the currently extracted data via $$.

   Solution

   module csv_naive;
   
   public type CSV = unit {
       rows: Row[];
   };
   
   type Row = unit {
       cols: bytes &until=b"\n" &convert=$$.split(b",");
   };
   

3. Without changing the actual parsing, can you change your grammar so the following output is produced? This can be done without explicit loops.

   $ spicy-driver csv_naive.spicy -f input.csv
   [[b"I", b"a", b"ABC"], [b"J", b"b", b"DEF"], [b"K", b"c", b"GHI"]]
   

   Hint 1

   You could add a unit hook for your top-level unit which prints the rows.

   on CSV::%done {
       print self.rows;
   }
   

   Since rows is a vector of units you still need to massage its data though ...

   Hint 2

   You can use a unit &convert attribute on your row type to transform it to its row data.

   Solution

   module csv_naive;
   
   public type CSV = unit {
       rows: Row[];
   };
   
   type Row = unit {
       data: bytes &until=b"\n" &convert=$$.split(b",");
   } &convert=self.data;
   
   on CSV::%done {
       print self.rows;
   }