File IO
Reading a Whole File
| rdf.hs |
| 1 | main = do |
| 2 | line <- readFile "/etc/group" |
| 3 | putStrLn line |
$ ghc --make rdf.hs
[1 of 1] Compiling Main ( rdf.hs, rdf.o )
Linking rdf ...
| $ ./rdf
root:x:0:root
bin:x:1:root,bin,daemon
daemon:x:2:root,bin,daemon
sys:x:3:root,bin,adm
adm:x:4:root,adm,daemon
tty:x:5:
disk:x:6:root
lp:x:7:daemon,lp
mem:x:8:
kmem:x:9:
wheel:x:10:root
mail:x:12:mail
news:x:13:news
uucp:x:14:uucp
man:x:15:
games:x:20:
gopher:x:30:
dip:x:40:
ftp:x:50:
lock:x:54:
nobody:x:99:
users:x:100:
utmp:x:22:
utempter:x:35:
floppy:x:19:
vcsa:x:69:
nscd:x:28:
slocate:x:21:
tcpdump:x:72:
gnokii:x:499:
dbus:x:81:
polkituser:x:87:
pulse:x:498:
pulse-rt:x:497:
pulse-access:x:496:
smolt:x:495:
rpcuser:x:29:
nfsnobody:x:494:
avahi:x:493:
wbpriv:x:88:
openvpn:x:492:
apache:x:48:
mailnull:x:47:
smmsp:x:51:
sshd:x:74:
haldaemon:x:68:
ntp:x:38:
gdm:x:42:
sbrandt:x:500:
jackuser:x:491:
hsqldb:x:96:
ubiqis:x:490:
postgres:x:26:
tomcat:x:91:
gold:x:489:
amie:x:488:
tgcdb:x:487:
torrent:x:486:
avahi-autoipd:x:485:
screen:x:84:
mysql:x:27:
rpc:x:32:
|
Reading a File Line by Line
| rdf2.hs |
| 1 | import System.IO |
| 2 | |
| 3 | doline fh = do |
| 4 | line <- hGetLine fh |
| 5 | putStrLn line |
| 6 | prlines fh |
| 7 | |
| 8 | prlines fh = do |
| 9 | eof <- hIsEOF fh |
| 10 | case eof of |
| 11 | True -> putStr "" |
| 12 | False -> doline fh |
| 13 | |
| 14 | main = do |
| 15 | fh <- openFile "/etc/group" ReadMode |
| 16 | prlines fh |
| 17 | hClose fh |
$ ghc --make rdf2.hs
[1 of 1] Compiling Main ( rdf2.hs, rdf2.o )
Linking rdf2 ...
| $ ./rdf2
root:x:0:root
bin:x:1:root,bin,daemon
daemon:x:2:root,bin,daemon
sys:x:3:root,bin,adm
adm:x:4:root,adm,daemon
tty:x:5:
disk:x:6:root
lp:x:7:daemon,lp
mem:x:8:
kmem:x:9:
wheel:x:10:root
mail:x:12:mail
news:x:13:news
uucp:x:14:uucp
man:x:15:
games:x:20:
gopher:x:30:
dip:x:40:
ftp:x:50:
lock:x:54:
nobody:x:99:
users:x:100:
utmp:x:22:
utempter:x:35:
floppy:x:19:
vcsa:x:69:
nscd:x:28:
slocate:x:21:
tcpdump:x:72:
gnokii:x:499:
dbus:x:81:
polkituser:x:87:
pulse:x:498:
pulse-rt:x:497:
pulse-access:x:496:
smolt:x:495:
rpcuser:x:29:
nfsnobody:x:494:
avahi:x:493:
wbpriv:x:88:
openvpn:x:492:
apache:x:48:
mailnull:x:47:
smmsp:x:51:
sshd:x:74:
haldaemon:x:68:
ntp:x:38:
gdm:x:42:
sbrandt:x:500:
jackuser:x:491:
hsqldb:x:96:
ubiqis:x:490:
postgres:x:26:
tomcat:x:91:
gold:x:489:
amie:x:488:
tgcdb:x:487:
torrent:x:486:
avahi-autoipd:x:485:
screen:x:84:
mysql:x:27:
rpc:x:32:
| See http://cgi.cse.unsw.edu.au/~dons/blog/2006/12/17.
The problem with this approach is that it betrays an imperative
way of thinking. Possibly the programmer was worried that readFile,
because it returns the entire contents of a file as a string, was
wasting memory. Not so. Haskell is smart enough to only read as
much of the file as it needs to.
Read a File Line by Line, Again
| rdfs3.hs |
| 1 | -- Split up a string into an array, using character c as a delimiter |
| 2 | split _ [] = [] |
| 3 | split c str = |
| 4 | (takeWhile (c /= ) str) : (split c (drop 1 (dropWhile (c /=) str))) |
| 5 | |
| 6 | -- Print out lines one by one |
| 7 | prlines [] = putStr "" |
| 8 | prlines (a:b) = do |
| 9 | putStrLn a |
| 10 | prlines b |
| 11 | |
| 12 | main = do |
| 13 | file <- readFile "/etc/group" |
| 14 | prlines(split '\n' file) |
$ ghc --make rdfs3.hs
[1 of 1] Compiling Main ( rdfs3.hs, rdfs3.o )
Linking rdfs3 ...
| $ ./rdfs3
root:x:0:root
bin:x:1:root,bin,daemon
daemon:x:2:root,bin,daemon
sys:x:3:root,bin,adm
adm:x:4:root,adm,daemon
tty:x:5:
disk:x:6:root
lp:x:7:daemon,lp
mem:x:8:
kmem:x:9:
wheel:x:10:root
mail:x:12:mail
news:x:13:news
uucp:x:14:uucp
man:x:15:
games:x:20:
gopher:x:30:
dip:x:40:
ftp:x:50:
lock:x:54:
nobody:x:99:
users:x:100:
utmp:x:22:
utempter:x:35:
floppy:x:19:
vcsa:x:69:
nscd:x:28:
slocate:x:21:
tcpdump:x:72:
gnokii:x:499:
dbus:x:81:
polkituser:x:87:
pulse:x:498:
pulse-rt:x:497:
pulse-access:x:496:
smolt:x:495:
rpcuser:x:29:
nfsnobody:x:494:
avahi:x:493:
wbpriv:x:88:
openvpn:x:492:
apache:x:48:
mailnull:x:47:
smmsp:x:51:
sshd:x:74:
haldaemon:x:68:
ntp:x:38:
gdm:x:42:
sbrandt:x:500:
jackuser:x:491:
hsqldb:x:96:
ubiqis:x:490:
postgres:x:26:
tomcat:x:91:
gold:x:489:
amie:x:488:
tgcdb:x:487:
torrent:x:486:
avahi-autoipd:x:485:
screen:x:84:
mysql:x:27:
rpc:x:32:
| |