[Nix-dev] haskell ide

[Mateusz Kowalczyk]

On 05/12/2014 10:15 PM, Vladimír Čunát wrote:
> On 05/11/2014 09:50 AM, Roelof Wobben wrote:
>> but can someone explain to me what holes and missing cases are.
> 
> I don't know that. GHC traditionally also implements optional 
> research-level features, so something from those, I presume.

Holes are things where you ask the compiler to tell you the type of the
expression that you need to fill in.

For example, if we denote holes with ‘?’:

foo :: [String] -> [Integer]
foo xs = map ? xs

The compiler might tell us that the hole is expecting an expression of
type ‘String -> Integer’. This is a new feature in GHC although Agda had
it for a while.

Prelude> let foo :: [String] -> [Integer]; foo xs = map _ xs in foo []




<interactive>:2:48:



    Found hole ‘_’ with type: String -> Integer



    Relevant bindings include



      xs :: [String] (bound at <interactive>:2:39)



      foo :: [String] -> [Integer] (bound at <interactive>:2:35)



      it :: [Integer] (bound at <interactive>:2:1)



    In the first argument of ‘map’, namely ‘_’



    In the expression: map _ xs
    In an equation for ‘foo’: foo xs = map _ xs


The reason it's coming up in an IDE thread is that you can do very nice
things with this: for example, agda-mode for emacs lets you jump between
holes, try to find the things to insert automatically, refine into
smaller chunks etc.


Missing cases is probably referring to the ability of the compiler to
tell you whether your patterns are complete:

Prelude> :set -Wall
Prelude> let f [] = "Hello" in f []

<interactive>:6:5: Warning:
    Pattern match(es) are non-exhaustive
    In an equation for ‘f’: Patterns not matched: _ : _
"Hello"

I'm unsure why this is coming up in an IDE thread except maybe with the
IDE being able to highlight such cases or something. Case splitting is a
much more useful feature I feel.

> Anyway, I don't think there's any need for you to care about such things 
> (yet) if you're just starting with Haskell. Haskell2010 contains quite a 
> lot to grasp by itself.

Actually, programming with holes can make for a great development tool
for beginners and experts alike (combined with ScopedTypeVariables) and
ability to report missing cases is invaluable.

I do agree that OP shouldn't be seeking a fully-featured IDE which will
handle all the Cabal stuff and everything around it behind-the-scenes
because when it breaks, he won't know what to do.

Honestly I'm not quite sure why this thread ended up on nix-dev rather
than haskell-cafe or haskell-beginners.

> 
> Vlada
> 
> 
> 
> 
> _______________________________________________
> nix-dev mailing list
> nix-dev at lists.science.uu.nl
> http://lists.science.uu.nl/mailman/listinfo/nix-dev
> 


-- 
Mateusz K.