[Haskell-cafe] Documenting the impossible

Henning Thielemann lemming at henning-thielemann.de
Sat Jun 14 17:01:28 EDT 2008

On Sat, 14 Jun 2008, Neil Mitchell wrote:

> Hi
>>  How about a {-# IMPOSSIBLE #-} pragma that documents the fact that a
>> particular point in the program *should* be unreachable?
> Why not make it a function taking a string and returning a value of
> any type. Then we can keep our language and not break various
> parsing/type checking properties and rules on pragmas. We can even
> define it:
> impossible = error
> Now you can use tools like Catch and Reach to ensure it is impossible.
> I think it would be nice to distinguish between:
> * error - inserted by the compiler


> * impossible - the programmer knows this can't occur

This is an error. However the programmer adds a message because he might 
be wrong.

> * abort - deliberate aborting because the user made some mistake.

This is an exception. The signature of a function must reflect this by a 
Maybe, Either type etc.


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