[Haskell-cafe] Trapped by the Monads

[Michael Walter]

Compare:

  int *p=...;
  int x=*p;

and:

  let
    p = ...
    Just x = p

So actually, there is few difference between dereferencing a pointer
without checking for 0, and extracting the Maybe value without
handling Nothing, apart from that it leads to undefined behavior in C
which in fact isn't really a point against "hybrid variables".

On 9/20/05, Lennart Augustsson <lennart at augustsson.net> 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.)
> 
>         -- Lennart
> 
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