Hello. I am having two problems. I have a program that reads two texts file
and writes a new text with its info. So I have done so that if main file
doesn't exist I just output an error string but my problem is when it comes
to the second file. This file is not exactly needed so if it is not there,
the program should just ignore the file. I tried doing that but if the
second file is there I also get an error saying that it couldn;t find the
file. So is it possible to check whether a file exists or not outputting and
empty string if not or the actual data from it if so without giving an error
and making the program not run??
For example
main = do
mainfile <- readFile "xxxxx.txt"
secondfile <- readMyFile "yyyy.txt"
writeFile "test.txt" mainfile++secondfile
readMyFile file = do
list <- readFile file
if list == "" then return "" else return list
Apparently this doesn't work and if file doesn't exist it still gives me an
error.
Another problem I ran into is with IO[String] and [String].
I have a function like
foo = do
nf <- readFile "xxx.txt"
return (lines nf)
Then when I try using foo like a [String] in other programs I run into type
error. How can I turn this IO[String] into a [String] so I don't actually
have to open this file in the main and then use it as an argument of another
function running "lines' on main method????
Best Regards
Alex
----- Original Message -----
From: "Ron de Bruijn"
Why? It's not always necessary to go for maximum speed; do you have a reason for needing it in this case?
Almost any operation in my program works on array's and before the program terminates, there have been an awful lot of operations on it. If there is an other kind of array that mutates and doesn't create a copy of it when changing a value and isn't more than 1,5 times as slow as an array in C++ then that should also be ok. The program would run in a scale of hours or days. And if it's twice as fast, than I could calculate two times as much.
Constructing mutable arrays
newArray :: (MArray a e m, Ix i) => (i, i) -> e -> m (a i e)
Builds a new array, with every element initialised to undefined.
No; that's newArray_. (newArray bounds x) builds an array with every element initialized to x.
Reading and writing mutable arrays
readArray :: (MArray a e m, Ix i) => a i e -> i -> m e
Read an element from a mutable array
writeArray :: (MArray a e m, Ix i) => a i e -> i -> e -> m () Write an element in a mutable array
I see it takes two arguments: a tuple of an index type (I use Int, so for example (0,1) 0 for the lower bound, 1 for the upper bound.),
I assume you're talking about newArray here, right? If so, you're correct.
but the e (according to "GHC documentation" it says the "element type"). Well the type of my elements should be my own datatype myDataType, but I can't even get it done to create simple mutable array's of Int or Bool.
I think you mean `above'. In any case, could you show us some code you have that's not working? That'll make it much easier for us to answer your questions.
Well I don't have really code, because I want to begin programming it.
I will restate my goal: I want to be able to read, update and create arrays in Haskell of type myDataType(suppose my constructor function is Con Int String Int on a way that's fast, preferabily the fastest method.
Is it OK to say that in Haskell any value that is put from an extern source of the program (reading a file, reading stdin etc.) to a really pure function, must be in a do notation (essentially being a Monadic operation)?
Yes. More specifically, they don't need to use do-notation (although if you don't know why not, don't worry about it), but they do have to be monadic. Specifically, they have to use the IO monad.
I know it's also possible to use (>>=) and a function that works on values of the bound variable.
So to put it more concrete: In any Haskell program that does IO there is always some function that has this form:
do x<-someSource
putBoundVariableToSomeOtherFunctionThatDoesMonadicOperations
(restOfPureFunctionalProgramForExample x)
No.
Why not?
Greets Ron
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