Re: [Haskell-cafe] What *not* to use Haskell for

On Wed, 2008-11-12 at 10:50 -0800, Don Stewart wrote:

tux_rocker:

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2008/11/11 Dave Tapley :

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So I should clarify I'm not a troll and do "see the Haskell light". But one thing I can never answer when preaching to others is "what does Haskell not do well?"

Let's say something controversial: I think that Haskell's type system gets in your way when you're writing one-shot scripts that don't need to conform to the highest correctness standards. Imagine typing a command at the shell prompt and getting the sort of abstract error message that Haskell compilers give every now and then, like:

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whyerror.lhs:36:25: Ambiguous type variable `a' in the constraint: `Arrow a' arising from use of `>>>' at whyerror.lhs:36:25-27 Possible cause: the monomorphism restriction applied to the following: liftA2' :: forall b a1 b1 c. (a1 -> b1 -> c) -> a b a1 -> a b b1 -> a b c (bound at whyerror.lhs:36:1) unsplit' :: forall a1 b c. (a1 -> b -> c) -> a (a1, b) c (bound at whyerror.lhs:34:1) split' :: forall b. a b (b, b) (bound at whyerror.lhs:33:1) Probable fix: give these definition(s) an explicit type signature or use -fno-monomorphism-restriction

You don't want to be bothered by such monstrosities (yes, they are, even though many of you may not see it because of years of conditioning) when you're just hacking up a simple script to make a catalog of your MP3 collection / check for patterns in log files / whatever.

Why are you using Arrows in your one shot scripts?

Because a generic operator like (>>>) is a) More likely to exist, and b) easier to remember than a special case operator like --- actually, I don't think GHC does come with a standard version of (>>>) type-specialized to functions, only a version of (<<<). Or if it does, I can't remember it.

Do you use Arrows in your shell scripts?

I use pipelines. Frequently. I even type them up at the command line. Beyond that, I was considering the pipeline uses -l snippet_calculate\\b | perl -lne 'chomp; print $_ unless qx{svn st | grep $_}' (`uses' is a recursive grep-like shell function I have). The use of perl immediately suggested that it could profitably be re-written in Haskell (actually, in a similar FP language I've been designing --- not relevant); my translation included a line something like interactiveM $ filterM $ comp >>> \ x -> So yeah, I use arrows in my shell scripts. Who doesn't? (On the other hand, I'm actively designing an FP shell-like language...) jcc