[Haskell-beginners] Re: Showing control characters
byorgey at seas.upenn.edu
Tue Nov 9 22:56:33 EST 2010
On Tue, Nov 09, 2010 at 11:44:42AM -0800, Russ Abbott wrote:
> I wasn't at a computer with Haskell when I wrote the earlier message.
> Here's how it seems to work.
> *data *MyType = MyType
> *instance *Show MyType *where*
> show MyType = "a\nb"
There's nothing too magical going on here. Remember that show just
converts a value to a String. How that String is then displayed
depends on what you do with it, not with the definition of show.
> Load the above file. Then:
> > MyType -- This does what I want it to do after all.
ghci essentially does "putStrLn . show" on the results of expressions
typed at the prompt. So the call to show results in the string
consisting of an a, a newline, and a b, and putStrLn prints that
string to the screen (with the newline interpreted appropriately).
> > show MyType
'show MyType' is an expression of type String. Doing putStrLn . show
on it causes show to be called on the string, which converts it to a
form surrounded by quotes and with control characters escaped. The
resulting string is then printed to the screen.
> > putStrLn (show MyType)
This is the same as the first example, since IO actions entered at the
ghci prompt are simply run.
> > print MyType
This is also the same, since print = putStrLn . show.
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