Lennart Augustsson wrote:
Mark Carter wrote:
The typical example in C is:
mem = malloc(1024)
Malloc returns 0 to indicate that memory cannot be allocated, or a
memory address if it can. The variable mem is a so-called hybrid
variable; it crunches together 2 different concepts: a boolean value
(could I allocate memory?) and an address value (what is the address
where I can find my allocated memory).
It's considered a bad idea because it makes it easy for programmers
to use the value inappropriately - witness the number of programmers
who pass in 0 as a memory location.
This is a bad idea in C, because you cannot force programmers to test
the return value properly.
The Maybe type in Haskell is a good idea, because you must test the
a Maybe value to extract the real value. (Using the Maybe monad this
can be hidden.)
Yes, I didn't mean to imply that Haskell had got its ideas fundamentally
wrong. I think that the main thrust of the argument is that in something
like C, hybrid variables make it easy for a programmer to inadvertently
use them wrongly, as opposed to just be lazy. I suppose there's some
element of debate on the matter: for example, what C programmer doesn't
honestly know that memory allocation can fail. And for an unfamiliar
function, why doesn't the programmer read the documentation to find out
what constitutes valid and invalid return values.
OTOH, I think Charles Moore is quoted as saying that if he want to add 1
to the letter A, then he didn't want the programming language to stop
him. Which is quite a contrast to Haskell and its notions on safety. I'm
not flamebaiting, you understand, I'm just pointing out the various
viewpoints to the argument.
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