How can I express this Python for loop in Haskell?

Question

Sometimes when I want to use wget, I just end up printing a bunch of lines with Python like so:

>>> for i in range(25):
...   print "http://www.theoi.com/Text/HomerOdyssey", i, ".html"
... 
http://www.theoi.com/Text/HomerOdyssey 0 .html
http://www.theoi.com/Text/HomerOdyssey 1 .html
http://www.theoi.com/Text/HomerOdyssey 2 .html
http://www.theoi.com/Text/HomerOdyssey 3 .html
http://www.theoi.com/Text/HomerOdyssey 4 .html
http://www.theoi.com/Text/HomerOdyssey 5 .html
http://www.theoi.com/Text/HomerOdyssey 6 .html
http://www.theoi.com/Text/HomerOdyssey 7 .html
http://www.theoi.com/Text/HomerOdyssey 8 .html
http://www.theoi.com/Text/HomerOdyssey 9 .html
http://www.theoi.com/Text/HomerOdyssey 10 .html
http://www.theoi.com/Text/HomerOdyssey 11 .html
http://www.theoi.com/Text/HomerOdyssey 12 .html
http://www.theoi.com/Text/HomerOdyssey 13 .html
http://www.theoi.com/Text/HomerOdyssey 14 .html
http://www.theoi.com/Text/HomerOdyssey 15 .html
http://www.theoi.com/Text/HomerOdyssey 16 .html
http://www.theoi.com/Text/HomerOdyssey 17 .html
http://www.theoi.com/Text/HomerOdyssey 18 .html
http://www.theoi.com/Text/HomerOdyssey 19 .html
http://www.theoi.com/Text/HomerOdyssey 20 .html
http://www.theoi.com/Text/HomerOdyssey 21 .html
http://www.theoi.com/Text/HomerOdyssey 22 .html
http://www.theoi.com/Text/HomerOdyssey 23 .html
http://www.theoi.com/Text/HomerOdyssey 24 .html
>>>

I can paste that output into a new file, remove the spaces, and use wget -i.

But I am sick of Python.

I want to learn Haskell.

Despite spending 10 minutes trying to do the same thing from ghci, I am no further forward.

This is what my attempts looked like:

alec@ROOROO:~/oldio$ ghci
GHCi, version 7.0.4: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Prelude> putStrLn

<interactive>:1:1:
    No instance for (Show (String -> IO ()))
      arising from a use of `print'
    Possible fix:
      add an instance declaration for (Show (String -> IO ()))
    In a stmt of an interactive GHCi command: print it
Prelude> putStrLn "hey"
hey
Prelude> putStrLn "hey" [1..10]

<interactive>:1:1:
    The function `putStrLn' is applied to two arguments,
    but its type `String -> IO ()' has only one
    In the expression: putStrLn "hey" [1 .. 10]
    In an equation for `it': it = putStrLn "hey" [1 .. 10]
Prelude> putStrLn "hey" snd [1..10]

<interactive>:1:1:
    The function `putStrLn' is applied to three arguments,
    but its type `String -> IO ()' has only one
    In the expression: putStrLn "hey" snd [1 .. 10]
    In an equation for `it': it = putStrLn "hey" snd [1 .. 10]
Prelude> putStrLn "hey" $ snd [1..10]

<interactive>:1:1:
    The first argument of ($) takes one argument,
    but its type `IO ()' has none
    In the expression: putStrLn "hey" $ snd [1 .. 10]
    In an equation for `it': it = putStrLn "hey" $ snd [1 .. 10]
Prelude> "hello"
"hello"
Prelude> "hello" ++ "world"
"helloworld"
Prelude> "hello" ++ [1..10] ++ " world"

<interactive>:1:16:
    No instance for (Num Char)
      arising from the literal `10'
    Possible fix: add an instance declaration for (Num Char)
    In the expression: 10
    In the first argument of `(++)', namely `[1 .. 10]'
    In the second argument of `(++)', namely `[1 .. 10] ++ " world"'
Prelude> "hello" ++ print [1..10] ++ " world"

<interactive>:1:12:
    Couldn't match expected type `[Char]' with actual type `IO ()'
    In the return type of a call of `print'
    In the first argument of `(++)', namely `print [1 .. 10]'
    In the second argument of `(++)', namely
      `print [1 .. 10] ++ " world"'
Prelude> print [1..10]
[1,2,3,4,5,6,7,8,9,10]
Prelude> map ("hello") [1..10]

<interactive>:1:6:
    Couldn't match expected type `a0 -> b0' with actual type `[Char]'
    In the first argument of `map', namely `("hello")'
    In the expression: map ("hello") [1 .. 10]
    In an equation for `it': it = map ("hello") [1 .. 10]
Prelude> greeting :: String --> Int  -> [String, Int]

<interactive>:1:39: parse error on input `,'
Prelude> greeting :: String --> Int  -> [(String), (Int)]

<interactive>:1:41: parse error on input `,'
Prelude> greeting :: String -> Int  -> [(String), (Int)]

<interactive>:1:40: parse error on input `,'
Prelude> greeting :: String -> Int  -> [(String) (Int)]

<interactive>:1:1: Not in scope: `greeting'
Prelude> foreach [1..24] print

<interactive>:1:1: Not in scope: `foreach'
Prelude> import Data.IORef
Prelude Data.IORef> foreach [1..24] print

<interactive>:1:1: Not in scope: `foreach'
Prelude Data.IORef> foreach = flip mapM_

<interactive>:1:9: parse error on input `='

As you have discovered, Haskell I/O isn't a 10 minute topic... — Luis Casillas
– Luis Casillas, Commented May 22, 2012 at 15:20
@sacundim putStrLn "hey" vs putStrLn "hey" [1..10] isn't an IO issue -- he's not got the control structures down yet. — Don Stewart
– Don Stewart, Commented May 22, 2012 at 15:27
You can avoid getting the spaces in Python with print "http://www.theoi.com/Test/HomerOdyssey{0}.html".format(i) (among many other ways) — sdcvvc
– sdcvvc, Commented May 22, 2012 at 21:39

Don Stewart · Accepted Answer · 2012-05-22 14:17:46Z

13

for i in range(25):
...   print "http://www.theoi.com/Text/HomerOdyssey", i, ".html"

becomes:

import Control.Monad

so that we can:

 forM_ [1..25] $ \i ->
     putStrLn $ "http://www.theoi.com/Text/HomerOdyssey" ++ show i ++ ".html"

answered May 22, 2012 at 14:17

Don Stewart

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4 Comments

Marcin Over a year ago

It might help to explain what $ is.

Don Stewart Over a year ago

@Marcin stackoverflow.com/questions/940382/… and stackoverflow.com/questions/3030675/…

Marcin Over a year ago

It might help to explain it in your answer.

Don Stewart Over a year ago

The $ operator is for avoiding parenthesis. It brackets anything to the right of it. It avoids Lisp-like clippings.

dave4420 · Accepted Answer · 2012-05-22 14:24:55Z

9

mapM_ (\i -> putStrLn (concat ["http://www.theoi.com/Text/HomerOdyssey", show i, ".html"])) [0..24]

As a bonus, this doesn't print any spaces.

Now some theory:

putStrLn is a function that takes a single argument. Python, Perl, etc will slurp up all the arguments you give to print and turn it into a single string. In Haskell you have to do that yourself.
mapM_ takes two arguments. Second a list, and first a function which mapM_ passes each element of the list to in turn. The function we've passed here is an anonymous function (like a lambda in Python).

edited May 22, 2012 at 14:24

answered May 22, 2012 at 14:17

dave4420

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Comments

sdcvvc · Accepted Answer · 2012-05-22 22:28:18Z

3

Consider using a list comprehension:

mapM_ putStrLn ["http://www.theoi.com/Text/HomerOdyssey" ++ show i ++ ".html" | i <- [0..24]]

answered May 22, 2012 at 22:28

sdcvvc

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Comments

John L · Accepted Answer · 2012-05-23 12:04:19Z

This isn't exactly an answer, but it's too long for a comment. I think a few words about ghci will help a great deal.

In ghci, you can use :t and :info to show the type of something, and information about something, respectively.

Prelude> :t putStrLn
putStrLn :: String -> IO ()
Prelude> :info putStrLn
putStrLn :: String -> IO ()     -- Defined in `System.IO'
Prelude> :t putStrLn "Hello, World"
putStrLn "Hello, World" :: IO ()

:t is useful because you can use it to show the type of arbitrary expressions, which :info won't. But :info can provide information about things that don't have types, such as types themselves or type constructors:

Prelude> :info IO
newtype IO a
  = GHC.Types.IO (GHC.Prim.State# GHC.Prim.RealWorld
                  -> (# GHC.Prim.State# GHC.Prim.RealWorld, a #))
    -- Defined in `GHC.Types'
instance Monad IO -- Defined in `GHC.Base'
instance Functor IO -- Defined in `GHC.Base'

When you're trying to figure out how to make ghci accept something, :t can be very useful.

Next, realize that the ghci prompt gives you line-by-line do-notation, in IO. If you enter something of type IO x, that statement will be evaluated when you press enter. If you want to bind that x, use do-notation arrows. If you want to create a new binding (i.e. create a new function or identifier), use let. If you type in an expression that isn't IO, ghci will attempt to show it to you, which won't work if a Show instance isn't defined.

Prelude> putStrLn "Hello, World"
Hello, World
Prelude> :t getChar
getChar :: IO Char
Prelude> x <- getChar
yPrelude> 
Prelude> show x
"'y'"
Prelude> let g = 10 :: Int
Prelude> :t g
g :: Int
Prelude> show g
"10"
Prelude> g
10

Note that show g and g aren't the same. You can probably figure out why from the behavior I outlined above.

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