To my suggestion on the error messages
Ketil Malde <[EMAIL PROTECTED]> writes
> [..]
> e.g instead of Hugs' Prelude head:
>
> head :: [a] -> a
> head (x:_) = x
>
> we can use:
>
> head :: [a] -> a
> head [] = error "Error: taking the 'head' of an empty list"
> head (x:_) = x
The Haskell implementation I use reports something of this sort.
But this does not help.
Also "head []" is the only error that may happen to `head'.
I wrote that, generally, for the Prelude Functions, it would help in
error messages to print some part of value and type expression.
But with head [], I had, probably, mistaken.
----------------------------------------------
Because the program for `head' itself cannot find the type of `[]';
this may hope to do other functions, where `head' is called.
Probably, we cannot improve the function `head' itself.
(?)
But the things like take (-1) [(1,'b'),(2,'a')], (1)
let x = 0 in 0 ^ 0, (2)
y % 0 (3)
could produce better messages.
Here x :: Num a => a, y :: Integral b => b for some a,b.
One needs the values and type expression to be displayed.
The latter is desirable too. Because `0',`1' may denote zero, unity of
many different Num instances.
If
the data were of Show instance,
class Show included the operation showType, with the
reasonable standard instances,
Show was a superclass of Num,
then, one could express such type displaying in Haskell language for
the cases (2), (3).
With `take', it is harder.
We need the possibility of overlapping values to define
take :: Show a => Int -> [a] -> [a]
that overlaps with more generic take :: Int -> [a] -> [a]
The first specialization would produce better error report at the
run-time.
> Note that take already does bomb out if given a negative argument.
And I take this `take' only as example,
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for there was earlier the discussion in this mail list on how to treat
negative or too large index in `take'.
------------------
Sergey Mechveliani
[EMAIL PROTECTED]
