Here is how I want print to be in Haskell
print :: (a->b) -> (a->b)
with print = id, but the following "side effect":
- I want to call the print function today, and get the value tomorrow.
On Fri, 28 Dec 2007 12:03:04 +0200, apfelmus
Cristian Baboi wrote:
http://en.wikipedia.org/wiki/First-class_object The term was coined by Christopher Strachey in the context of “functions as first-class citizens” in the mid-1960's.[1] Depending on the language, this can imply: 1. being expressible as an anonymous literal value 2. being storable in variables 3. being storable in data structures 4. having an intrinsic identity (independent of any given name) 5. being comparable for equality with other entities 6. being passable as a parameter to a procedure/function 7. being returnable as the result of a procedure/function 8. being constructable at runtime 9. being printable 10. being readable 11. being transmissible among distributed processes 12. being storable outside running processes I'll guess that 5,9,12 does not apply to Haskell functions.
Exactly, together with 10 and 11 (when the distributed processes are on different machines).
But there is good reason that those things can't be done in Haskell. With extensional equality (two functions are considered equal if they yield the same result on every possible argument) number 5 is undecidable. Similarly, there cannot be functions
print :: (Int -> Int) -> String compile :: String -> (Int -> Int)
with
compile . print = id
A print function based on an intensional representation (assembly, byte code, etc.) would have to distinguish extensionally equal functions
print f ≠ print g although f = g
which is not allowed.
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