error code 1200 from pig shell

[Haitao Yao]

hi, all
        here's my pig script:
grunt> A = load '/tmp/test_file' using PigStorage(',') as (uid:long, 
payload:bytearray, ts:long, type:int);
grunt> B = foreach A generate COUNT(A);
2012-02-08 17:55:44,647 [main] ERROR org.apache.pig.tools.grunt.Grunt - ERROR 
1200: Pig script failed to parse:
<line 4, column 29> Invalid scalar projection: A : A column needs to be 
projected from a relation for it to be used as a scalar
Details at logfile: /tmp/pig_1328694479593.log

what does  ERROR 1200 mean?

How can I find all the meanings of error codes in Pig?

thanks.