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Why shouldn't I mix tabs and spaces?
(1 answer)
Closed 6 years ago.
Why is the following code not correct ? i get this error: Last generator in do {...} must be an expression?
main = do putStrLn "What is 2 + 2?"
x <- readLn
if x == 4
then putStrLn "You're right!"
else putStrLn "You're wrong!"
You are mixing tabs and spaces: the second and fifth line contain tabs, while the third and fourth don't.
The Haskell compiler probably expands tabs to a different number of spaces than your editor and what looks correctly indented in the editor looks messed up to the compiler.
Best avoid mixing tabs and spaces and only use one of them for indentation.
You had tabulations instead of spaces. After you have pasted the code into stackoverflow there are only spaces and everything is working.
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Haskell writes '\n' instead of a newline
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Closed 2 years ago.
I'm trying to write a program that writes the pound sign '£' in Haskell, but it outputs '\163' whenever I try to use it. I'm guessing that this is some alphanumeric code, but how do I get it to display what I want it to? I'm writing to the console, when calling a function that returns '£'.
Thank you.
This was solved by using putStrLn, because print and show do not allow for non-ASCII characters to be shown.
I just wrote my first Haskell program, but there is an error that I cannot understand. I think it is right because I just wrote it like the example from a book. Could anyone help me please?
main = do
putStrLn "Hello, what's your name?"
name <- getLine
putStrLn ("Hey" ++ name ++ ", nice to meet you!")
The error message is:
parse error on input 'putStrLn'
It is strange.
Though it's impossible to tell from your posted code because SO converts tabs to spaces at least some of the time, the problem is likely that you input a literal tab character before putStrLn instead of four spaces as you did for the other two lines in your do block, or vice versa.
All of the statements in a do block must start with the exact same whitespace, and not just appear to line up visually. If you're using a text editor that can display literal tabs in a special way, set it up to do so; it will save you some headaches.
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Closed 8 years ago.
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In this Script,
approximation :: Int -> (String, Int)
approximation x
| (x<20000) && (19000<=x) && (numDigits<x) = (text1, x-numDigits)
| (x<20000) && (19000<=x) && (numDigits>x) = (text1, numDigits-x)
| (x<19800) && (x>=19700) && (numDigits<x) = (text2, x-numDigits)
| (x<19800) && (x>=19700) && (numDigits>x) = (text2, numDigits-x)
| otherwise = ("far from no. of Digits", 0)
where
text1 = "at 1000th place of no. of reallyBig, abosolute error="
text2 = "at 100th place of no. of reallyBig, nearly Exact, absolute error"
I inputted 2 definitions: text1, text2 for the Function approximation. However, the compiler GHCI said there is a Parse error on input '=' in text2. I was confused by the problem.
You have mixed tabs and spaces for your indentation. This is a bad plan, because your editor and ghc can think about tabs quite differently. I think your editor is displaying tabs as (up to) 4 characters, whereas ghc thinks of tabs as (up to) 8 spaces. I'll write <--> for a tab and . for a space in your last two lines:
<-->....text1 = "at 1000th place of no. of reallyBig, abosolute error="
<--><-->text2 = "at 100th place of no. of reallyBig, nearly Exact, absolute error"
Which is how your editor displays it. If I put ghc's 8 space tabs in, you get
<-------->....text1 = "at 1000th place of no. of reallyBig, abosolute error="
<--------><-------->text2 = "at 100th place of no. of reallyBig, nearly Exact, absolute error"
and you get the parse error.
It's easiest if you stick to spaces. Change your editor's settings.
If you use just spaces, you can't get this problem, because your editor has to show it the way the compiler thinks about it.
My editor lets me specify that when I press tab, it should insert the number of spaces that a tab would show as, so I use that, which is safe for a tabstop of 4. If your editor can do that, use that option. (If not, consider getting a cleverer editor for when you're programming.
My editor also has auto indent and outdent, where the next line copies the whitespace indentation of the previous line - this avoids the problem. Turn this on if your editor supports it, because it saves you effort and you're less likely to get the parse error. (When I then press backspace, my editor deletes back to the previous level of indentation, which is nice.)
Almost all editors can change how they display tabs. If you can't get it to use spaces for tabs, you should change the tabstop to be 8, because that matches ghc, and you're much less likely to get this error, but you're still better off using spaces.
here is the code which to my mind shouldn't cause any issue but for some reason does?
program = expr8
<|> seqOfStmt
seqOfStmt =
do list <- (sepBy1 expr8 whiteSpace)
return $ if length list == 1 then head list else Seq list
I get 3 errors all in respect to 'list' not being in scope?
It's probably blatantly obvious what is going wrong but I can't figure out why
If there are any alternatives to this I would greatly like to hear them !
Thanks in advance,
Seán
Your final line uses a tab character for indentation, while the other lines use spaces only.
You have tabs set to four spaces in your editor, but ghc uses eight character tab stops (just as terminals do).
Therefore your return line is parsed as a continuation of the previous line, and list is not yet in scope.
One easy way to fix this is to refrain from using tabs: use spaces only.
Once you've fixed that, your next error will probably be a type error: head list and Seq list have different types (unless perhaps you have redefined head for some reason). It's not clear why you want to treat the list differently if it contains only a single element.
This question already has answers here:
Closed 10 years ago.
Possible Duplicate:
Python: Split string with multiple delimiters
Convert django Charfield “\t” to tab
I have written a python code in Eclipse which takes delimiters as an argument. When I do
print "Hello",delimiter, "All".
This generates --> Hello \t All, whereas if I overwrite the delimiter with delimiter = '\t' within the code, I get the right output Hello All. I wonder what is the difference? I hope this not just the eclipse thing.
The problem is that what is being passed in from the command line is actually a string of length two "\\t" and not a tab character. You can do the following to your delimiter
delimiter.decode("string_escape"))
that should convert the string '\\t' into '\t'. The answer comes from a duplicate questions here