As for head, I think it's fine that it throws an error because it is specified to be defined for only non-empty lists.
But surely it is better to encode this fact in the type system by useing a separate type for non-empty lists.
Argh, no! Violating the precondition of head is a bug in the caller, I want it to crash,
Again by encoding this at the type level the compiler can guarantee that that it is not going to crash or return a compile time error... also by forcing the programmer to deliberatly cast to a non-empty list you make explicit the 'non-emptyness'... also the type signature of head contains the 'contract' that the list must be non empty.
A mechanism for a function to report the caller site would obliviate the need for all this ugliness
Unfortunately the cost of this is prohabative as the call stack would have to contain backtrace information. Also due to lazy evaluation the 'call site' may not be what you expect. I guess we just have to disagree I find very little cause to use head. Here's what John Meacham had to say: ----------------------------------- Note that pattern matching rather than deconstruction functions have a number of benefits, not just relating to error messages, consider two functions which use the head of their argument. f xs =3D ... head xs ...=20 g (x:_) =3D ... x ... now, g is superior to f in several ways,=20 1) the error message generated if the pattern fails will have the file name, line number and name of function 2) everything but the head of xs may be garbage collected right away since there is no reference to the rest of the list! This can cure some nasty space leaks and is vital in certain cases.=20 3) even the simplest compiler will realize g is stirct in its first argument and take advantage of the numerous optimizations that entails.=20 A good compiler may figure out 2 and 3 with f, but it can't always, and w= hy take the chance it won't? All of these benefits apply to deconstructing more complicated types too, so using pattern matching for deconstruction is just a good habit to get into. -------------------------------------- Keean.