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Jose Morais

13 Oct 2003 13 Oct '03

12:54 p.m.

Hi, I am trying to something like f1 :: Int -> Int f1 x = f2 x f2 :: Int -> Int f2 x = 2 * x but before f2 returns its result I'd like it to print something like "The initial value is " ++ show x. How could I do this? Thank you

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Hal Daume III

13 Oct 13 Oct

12:56 p.m.

In general, you can't, not with these type signatures, since printing something is a side effect. If this is only for debugging purposes, you might try using trace from IOExts. You can use it in a nice fashion like:

...

f1 :: Int -> Int f1 x | trace ("The initial value is " ++ show x) False = undefined | otherwise = f2 x

In general, the 'trace ... False = undefined' thing is quite useful, but note that trace uses unsafePerformIO and very little is guarenteed about the output (especially the ordering might be different than you would expect...but it's usually fine for debugging purposes). On 13 Oct 2003, Jose Morais wrote:

...

Hi,

I am trying to something like

f1 :: Int -> Int f1 x = f2 x

f2 :: Int -> Int f2 x = 2 * x

but before f2 returns its result I'd like it to print something like "The initial value is " ++ show x.

How could I do this?

Thank you

_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

-- Hal Daume III | hdaume@isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume

Ferenc Wagner

14 Oct 14 Oct

5:36 a.m.

Hal Daume III writes:

...

...
f1 :: Int -> Int f1 x | trace ("The initial value is " ++ show x) False = undefined | otherwise = f2 x

In general, the 'trace ... False = undefined' thing is quite useful

How is it better than

...

f1 x = trace ("The initial value is " ++ show x) $ f2 x

? Feri.

Keith Wansbrough

7:24 a.m.

...

Hal Daume III writes:

...
...
f1 :: Int -> Int f1 x | trace ("The initial value is " ++ show x) False = undefined | otherwise = f2 x

In general, the 'trace ... False = undefined' thing is quite useful

How is it better than

...
f1 x = trace ("The initial value is " ++ show x) $ f2 x

It is much easier to use in real code. Consider f1 :: Foo -> Int f1 Alpha = 0 f1 (Beta i) = i f1 (Gamma x) = x*x f1 Delta = 4 f1 (Epsilon xs) = length xs Now which is easier: f1 :: Foo -> Int f1 Alpha = trace ("The initial value is " ++ show Alpha ) $ 0 f1 (Beta i) = trace ("The initial value is " ++ show (Beta i) ) $ i f1 (Gamma x) = trace ("The initial value is " ++ show (Gamma x) ) $ x*x f1 Delta = trace ("The initial value is " ++ show Delta ) $ 4 f1 (Epsilon xs) = trace ("The initial value is " ++ show (Epsilon xs)) $ length xs or f1 :: Foo -> Int f1 x | trace ("The initial value is " ++ show x) False = undefined f1 Alpha = 0 f1 (Beta i) = i f1 (Gamma x) = x*x f1 Delta = 4 f1 (Epsilon xs) = length xs ? --KW 8-)

Ferenc Wagner

10:13 a.m.

Keith Wansbrough writes:

...

...
Hal Daume III writes:

...
...
f1 :: Int -> Int f1 x | trace ("The initial value is " ++ show x) False = undefined | otherwise = f2 x

In general, the 'trace ... False = undefined' thing is quite useful

How is it better than

...
f1 x = trace ("The initial value is " ++ show x) $ f2 x

It is much easier to use in real code. Consider [...]

You went to extreme lengths answering my silly question. Thanks a lot, I see the point now. Feri.

Hal Daume III

10:20 a.m.

In addition to what Keith said, it's also guarenteed that the trace is evaluated as soon as the function is entered. - Hal On Tue, 2003-10-14 at 02:36, Ferenc Wagner wrote:

...

Hal Daume III writes:

...
...
f1 :: Int -> Int f1 x | trace ("The initial value is " ++ show x) False = undefined | otherwise = f2 x

In general, the 'trace ... False = undefined' thing is quite useful

How is it better than

...
f1 x = trace ("The initial value is " ++ show x) $ f2 x

?

Feri. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe -- Hal Daume III | hdaume@isi.edu "Arrest this man, he talks in maths." | www.isi.edu/~hdaume

Shawn P. Garbett

13 Oct 13 Oct

1:06 p.m.

On Monday 13 October 2003 11:54 am, Jose Morais wrote:

...

Hi,

I am trying to something like

f1 :: Int -> Int f1 x = f2 x

f2 :: Int -> Int f2 x = 2 * x

but before f2 returns its result I'd like it to print something like "The initial value is " ++ show x.

f1 :: Int -> IO Int f1 x = f2 x f2 :: Int -> IO Int f2 x = do putStrLn $ "The initial value is " ++ show x return (2 * x) The previously mentioned trace method is prefered for debugging because with this, one is now in the IO monad. Shawn

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Ferenc Wagner
Hal Daume III
Jose Morais
Keith Wansbrough
Shawn P. Garbett

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Jose Morais

Hal Daume III

Ferenc Wagner

Keith Wansbrough

Ferenc Wagner

Hal Daume III

Shawn P. Garbett

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