CsvError

Standard library class · Extends RuntimeError

A CsvError is what Emerald raises when CSV text can’t be read: a quoted cell that never closes, a header that names a column twice, a row with the wrong number of cells, or a cell that can’t become the kind of value the program asked for. The message says where the problem is.

struct Score {
    const name: String
    const points: Int
}

try {
    print(Csv.decode("name,points\nAda,twelve", as: List[Score]))
}
catch error: CsvError {
    print(error.message)
    print(error.line)
}

Output

line 2, column "points": expected a whole number, found "twelve"
2

A CsvError is a RuntimeError, so catch error: RuntimeError catches it too.

message: String

What went wrong, in words, naming the line, and the column where there is one.

line: Int?

The line of the text where the problem is, counting from 1, or nothing when it isn’t about one line, such as a column the text lacks altogether.

What the messages say

Message	When
line 2, column "points": expected a whole number, found "twelve"	A cell that can’t become its field’s type
line 3, column "points": this cell is empty	An empty cell for a field that needs a value
there is no column "points"; the columns are "name"	A column the struct needs that the text lacks
line 2 has 3 fields, but the header has 2	A row with the wrong number of cells, for decode and parse_records
the header names the column "name" twice	A header with a repeated or blank name
line 1: a quoted field that starts here never closes	A quote with no closing quote
a separator must be one character, not "ab"	A separator: that isn’t a single character

Avoiding the error

A CSV file usually comes from somewhere else, so mistakes in it are to be expected. Catch the error around the one call that reads it, and say what is wrong, using line to point the person to it:

const text = "name,points\nAda,12\nGrace"

try {
    print(Csv.parse_records(text))
}
catch error: CsvError {
    print("The file has a problem. #{error.message}")
}

Output

The file has a problem. line 3 has 1 fields, but the header has 2