csv.scm: Comma-Separated Value (CSV) Utilities in Scheme

Version 0.4, 2005-06-07, http://www.neilvandyke.org/csv-scm/

by Neil W. Van Dyke <neil@neilvandyke.org>

Copyright © 2004 - 2005 Neil W. Van Dyke. This program is Free Software; you can redistribute it and/or modify it under the terms of the GNU Lesser General Public License as published by the Free Software Foundation; either version 2.1 of the License, or (at your option) any later version. This program is distributed in the hope that it will be useful, but without any warranty; without even the implied warranty of merchantability or fitness for a particular purpose. See <http://www.gnu.org/copyleft/lesser.html> for details. For other license options and consulting, contact the author.

Introduction

The csv.scm Scheme library provides utilities for reading various kinds of what are commonly known as “comma-separated value” (CSV) files. Since there is no standard CSV format¹, this library permits CSV readers to be constructed from a specification of the peculiarities of a given variant. A default reader handles the majority of formats.

One of the main uses of this library is to import data from old crusty legacy applications into Scheme for data conversion and other processing. To that end, this library includes various conveniences for iterating over parsed CSV rows, and for converting CSV input to the SXML 3.0 Scheme XML format.

This library requires R5RS, SRFI-6, SRFI-23, and an integer->char procedure that accepts ASCII values.

Other implementations of some kind of CSV reading for Scheme include Gauche's text.csv module, and the Scsh's record-reader and related procedures. This library intends to be portable and more comprehensive.

Reader Specs

CSV readers are constructed using reader specs, which are sets of attribute-value pairs, represented in Scheme as association lists keyed on symbols. Each attribute has a default value if not specified otherwise. The attributes are:

newline-type: Symbol representing the newline, or record-terminator, convention. The convention can be a fixed character sequence (lf, crlf, or cr, corresponding to combinations of line-feed and carriage-return), any string of one or more line-feed and carriage-return characters (lax), or adaptive (adapt). adapt attempts to detect the newline convention at the start of the input and assume that convention for the remainder of the input. Default: lax
separator-chars: Non-null list of characters that serve as field separators. Normally, this will be a list of one character. Default: (#\,) (list of the comma character)
quote-char: Character that should be treated as the quoted field delimiter character, or #f if fields cannot be quoted. Note that there can be only one quote character. Default: #\" (double-quote)
quote-doubling-escapes?: Boolean for whether or not a sequence of two quote-char quote characters within a quoted field constitute an escape sequence for including a single quote-char within the string. Default: #t
comment-chars: List of characters, possibly null, which comment out the entire line of input when they appear as the first character in a line. Default: () (null list)
whitespace-chars: List of characters, possibly null, that are considered whitespace constituents for purposes of the strip-leading-whitespace? and strip-trailing-whitespace? attributes described below. Default: (#\space) (list of the space character)
strip-leading-whitespace?: Boolean for whether or not leading whitespace in fields should be stripped. Note that whitespace within a quoted field is never stripped. Default: #f
strip-trailing-whitespace?: Boolean for whether or not trailing whitespace in fields should be stripped. Note that whitespace within a quoted field is never stripped. Default: #f
newlines-in-quotes?: Boolean for whether or not newline sequences are permitted within quoted fields. If true, then the newline characters are included as part of the field value; if false, then the newline sequence is treated as a premature record termination. Default: #t

Making Reader Makers

CSV readers are procedures that are constructed dynamically to close over a particular CSV input and yield a parsed row value each time the procedure is applied. For efficiency reasons, the reader procedures are themselves constructed by another procedure, make-csv-reader-maker, for particular CSV reader specs.

— Procedure: make-csv-reader-maker reader-spec

Constructs a CSV reader constructor procedure from the reader-spec, with unspecified attributes having their default values.
For example, given the input file fruits.csv with the content:
          apples  |  2 |  0.42
          bananas | 20 | 13.69
     
a reader for the file's apparent format can be constructed like:
          (define make-food-csv-reader
            (make-csv-reader-maker
             '((separator-chars            . (#\|))
               (strip-leading-whitespace?  . #t)
               (strip-trailing-whitespace? . #t))))
     
The resulting make-food-csv-reader procedure accepts one argument, which is either an input port from which to read, or a string from which to read. Our example input file then can be be read by opening an input port on a file and using our new procedure to construct a reader on it:
          (define next-row
            (make-food-csv-reader (open-input-file "fruits.csv")))
     
This reader, next-row, can then be called repeatedly to yield a parsed representation of each subsequent row. The parsed format is a list of strings, one string for each column. The null list is yielded to indicate that all rows have already been yielded.
          (next-row) => ("apples" "2" "0.42")
          (next-row) => ("bananas" "20" "13.69")
          (next-row) => ()
     

Making Readers

In addition to being constructed from the result of make-csv-reader-maker, CSV readers can also be constructed using make-csv-reader.

— Procedure: make-csv-reader in [reader-spec]

Construct a CSV reader on the input in, which is an input port or a string. If reader-spec is given, and is not the null list, then a “one-shot” reader constructor is constructed with that spec and used. If reader-spec is not given, or is the null list, then the default CSV reader constructor is used. For example, the reader from the make-csv-reader-maker example could alternatively have been constructed like:
          (define next-row
            (make-csv-reader
             (open-input-file "fruits.csv")
             '((separator-chars            . (#\|))
               (strip-leading-whitespace?  . #t)
               (strip-trailing-whitespace? . #t)))))
     

Basic Input Conveniences

Several convenience procedures are provided for iterating over the CSV rows and for converting the CSV into a list. To the dismay of some Scheme purists, each of these procedures accepts a reader-or-in argument, which can be a CSV reader, an input port, or a string. If not a CSV reader, then the default reader constructor is used. For example, all three of the following are equivalent:

     (csv->list                                     string  )
     ==
     (csv->list (make-csv-reader                    string ))
     ==
     (csv->list (make-csv-reader (open-input-string string)))

— Procedure: csv-for-each proc reader-or-in

Similar to Scheme's for-each, applies proc, a procedure of one argument, to each parsed CSV row in series. reader-or-in is the CSV reader, input port, or string. The return

— Procedure: csv-map proc reader-or-in

Similar to Scheme's map, applies proc, a procedure of one argument, to each parsed CSV row in series, and yields a list of the values of each application of proc, in order. reader-or-in is the CSV reader, input port, or string.

— Procedure: csv->list reader-or-in

Yields a list of CSV row lists from input reader-or-in, which is a CSv reader, input port, or string.

Converting CSV to SXML

The csv->sxml procedure can be used to convert CSV to [SXML] format, for processing with various XML tools.

— Procedure: csv->sxml reader-or-in [row-element [col-elements]]

Reads CSV from input reader-or-in (which is a CSV reader, input port, or string), and yields an SXML representation. If given, row-element is a symbol for the XML row element. If row-element is not given, the default is the symbol row. If given col-elements is a list of symbols for the XML column elements. If not given, or there are more columns in a row than given symbols, column element symbols are of the format col-n, where n is the column number (the first column being number 0, not 1).
For example, given a CSV-format file friends.csv that has the contents:
          Binoche,Ste. Brune,33-1-2-3
          Posey,Main St.,555-5309
          Ryder,Cellblock 9,
     
with elements not given, the result is:
          (csv->sxml (open-input-file "friends.csv"))
          =>
          (*TOP*
           (row (col-0 "Binoche") (col-1 "Ste. Brune")  (col-2 "33-1-2-3"))
           (row (col-0 "Posey")   (col-1 "Main St.")    (col-2 "555-5309"))
           (row (col-0 "Ryder")   (col-1 "Cellblock 9") (col-2 "")))
     
With elements given, the result is like:
          (csv->sxml (open-input-file "friends.csv")
                     'friend
                     '(name address phone))
          =>
          (*TOP* (friend (name    "Binoche")
                         (address "Ste. Brune")
                         (phone   "33-1-2-3"))
                 (friend (name    "Posey")
                         (address "Main St.")
                         (phone   "555-5309"))
                 (friend (name    "Ryder")
                         (address "Cellblock 9")
                         (phone   "")))
     

Tests

The csv.scm test suite can be enabled by editing the source code file and loading Testeez.

History

Version 0.4 — 2005-06-07: Converted to Testeez. Minor documentation changes.
Version 0.3 — 2004-07-21: Minor documentation changes. Test suite now disabled by default.
Version 0.2 — 2004-06-01: Fixed strange case-related bug exhibited under Gauche 0.8 and 0.7.4.2 in csv-internal:make-portreader/positional. Thanks to Grzegorz Chrupa/la for reporting.
Version 0.1 — 2004-05-31: First release, for testing with real-world input.

Footnotes

[1] “The Comma Separated Value (CSV) File Format: Create or parse data in this popular pseudo-standard format,” Web page, viewed 2004-05-26, http://www.creativyst.com/Doc/Articles/CSV/CSV01.htm