[Haskell-beginners] A first try
apfelmus at quantentunnel.de
Mon Jun 27 15:32:37 CEST 2011
David Place wrote:
> On Jun 27, 2011, at 3:50 AM, Heinrich Apfelmus wrote:
>> Good point, but this actually shows that withFile should be even lazier. In particular:
>> * The file should not be opened until the string is demanded.
>> * The file should be closed as soon as the string has been demanded in full.
> Suppose the file is only partially demanded as in the case I quoted earlier.
> print10 =
> contents <- withFile "/usr/share/dict/words" ReadMode (\h -> hGetContents h)
> print $ take 10 contents
> The file would never be closed.
Note that my version of withFile has a different type and replaces
Regardless, in both cases the intention is that the file contents is
evaluated completely *inside* the scope of withFile . In particular, a
correct way to write your example would be
print10 = withFile' "/usr/share/dict/words" $ print . take 10
(This is completely analogous to how Iteratees require you to process a
file. The difference is that Iteratees expresses demand for the
characters inside a file via >>= whereas lazy IO expresses demand via
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