Re: [Haskell-cafe] Trapped by the Monads

There's a big difference. You can see you are doing something fishy, and the compiler can too, and it can warn you. -- Lennart Michael Walter wrote:

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 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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