rui yang wrote:
I want to print a function which itself have some functions as it's parameters and will return some functions as the results, and I want to print out the result, does anyone knows how to define the instance declaration of show class to this function type?
I don't know if it is possible to define a Show-instance for a function type, but you should not do so, because functions are usually "unshowable". If you have a function as a result you can only apply it to some further argument and (try to) show the result of that application.
Please, don't be so categorical, and if you confess that you don't know whether it is possible, then test first.
There are cases where you might create some complex data structures containing functional objects: for specific dispatching, simulating OO, for writing interpreters, etc.
Then you *MAY* need - if only for debugging - to look at your data. Of course you cannot "print the sine function", but it is easy to write, say, in Hugs,
instance Show (a->b) where show f = "<sorry, this is a function, I can't show it>"
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Moreover, if we get down from the Haskell crystal mountain, we might see languages where functional entities KEEP A LOT of secondary information, which permit to auto-document them in a more specific, communicative way. Look at Python functions...
Jerzy Karczmarczuk
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