default instance for IsString

[Yitzchak Gale]

J. Garrett Morris wrote:
> By this logic, head is "unsound", since head [] throws an error.
> Haskell types are pointed; Haskell computations can diverge.

Well, there are those who would actually agree with
that and banish 'head' and friends from the language.
But I'll agree with you here.

[As an aside - I'm finding that liberal use of Edward's
non-empty list type, found in the semigroups package,
solves many of those problems for me.]

But there are two crucial differences. First, head is
just a partial function, not basic language syntax.
Second, the divergence of head is constant and
well-known, and not dependent on the implementation
of a type class at particular types by various library
authors.

>  What happens after the computation diverges is
> irrelevant to type soundness.

Agreed. I'm not talking about type soundness, in the
technical sense. I'm talking about engineering
soundness.

Thanks,
Yitz