Hi Luke,
There are a couple of errors in your program.
First, the error message is about the use of File in the type class
constraint (... | FileSystem File). You pass it the File argument, but
it should be a type variable. The following signature removes this error:
readzipcode :: String *files -> ((Int, Char, Char), *files) | FileSystem
files
This expresses that your function readzipcode works for any type files
provided that it supports the operations defined in the type class
FileSystem.
Second, the three lines with freadi, freadc, and freadc are incompatible
with the fact that you've just opened a read-only file with sfopen. To
read from read-only files you need their twin functions sfreadi and
sfreadc.
Third, you need to 'thread along' the altered read-only file versions.
In your current function you would keep reading from the same position
of the same 'infile' value. The idiom for that is indeed the #-let
notation, and you'd get:
# (b1, i, infile) = sfreadi infile
# (b2, c1, infile) = sfreadc infile
# (b3, c2, infile) = sfreadc infile
Fourth, to avoid partial functions, I'd prefer an otherwise case at the
end of your function to catch failing reads, similar to the earlier
catch that you've written.
Hope this helps,
Peter
On 7/1/2014 12:14 AM, Luke Immes wrote:
Quick question; program will not compile; and I tried for hours to get
it to work.
Any help would be appreciated.
All I want is to read text from a file...; thanks.
clean 515>make
clm -h 20m zip -o zip
Compiling zip
Error [zip.icl,6,readzipcode]: FileSystemM:11 type context should
contain one or more type variables
make: *** [zip] Error 1
clean 515>
----------------
module zip
import StdEnv, StdFile, StdOrdList, StdList, StdInt, StdReal
inputfilename :== "zip.txt"
readzipcode :: String *File -> ((Int, Char, Char), *File) | FileSystem
File
readzipcode inputfilename files
# (readok, infile, files) = sfopen inputfilename FReadText files
| not readok = abort ("Cannot open input file: '" +++ inputfilename
+++ "'")
# (b1, i, files) = freadi infile
# (b2, c1, files) = freadc infile
# (b3, c2, files) = freadc infile
| (b1 && b2 && b3) = ((i, c1, c2), files)
Start :: *World -> ((Int, Char, Char), *World)
Start ((int, ch1, ch2), world) = readzipcode inputfilename world
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