[Haskell-beginners] problem with lazy IO

Yitzchak Gale gale at sefer.org
Wed Nov 9 19:40:32 CET 2011

Dennis Raddle wrote:
> I have the following code, which reads several thousand text files and
> creates some Map data. On Windows XP, it crapped out with the error
> "resource exhausted: too many open files." So I'm sure I need to make it
> stricter.


> I've never tried to do that before, so I'm not sure where the
> strictness is necessary.

When you use readFile, each file remains open until the entire
file has been read. When reading is delayed due to laziness,
the open file handles begin to pile up.

> Maybe all I need is to switch to strict ByteStrings?

That should solve your problem in this case, yes.
You might consider using Text instead of ByteString, though;
you are interpreting those values as text. Text has built-in
functions for parsing floating point numbers. With
ByteString, you'd have to change each line to a String.
I almost always prefer using Text or ByteString over
String nowadays anyway.

To solve the problem when using readFile from the
Prelude, you would need to make sure that each
file is read all the way to the end as you go along.
One trick sometimes used for that is to use the
evaluate function from Control.Exception to force
evaluation of the length of each file:

  b <- readFile s
  evaluate $ length b

That would cause the entire contents of the file
to be read into memory immediately and the
file to be closed, like the behavior of readFile
for strict ByteString and strict Text.


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