[Haskell-cafe] Editors for Haskell

Simon Peyton-Jones simonpj at microsoft.com
Sun Jun 4 07:53:21 EDT 2006


You probably know this, but your kind of application is a big reason
that we now make GHC available as a library.  (Just say 'import GHC'.)  

You shouldn't need to parse Haskell yourself: just call GHC's parser.
You get back a syntax tree with very precise location information that
can guide your editor (e.g. if you want to select a sub-exprssion).
Similarly, you can call the type checker.

All this is much harder with half-written programs, but it seems
acceptable to re-run the process at every keystroke, or at least at
every pause.


| -----Original Message-----
| From: haskell-cafe-bounces at haskell.org
[mailto:haskell-cafe-bounces at haskell.org] On Behalf Of
| Brian Hulley
| Sent: 30 May 2006 20:00
| To: Mathew Mills; Bjorn Bringert; Christopher Brown
| Cc: Walter Potter; haskell-cafe at haskell.org
| Subject: Re: [Haskell-cafe] Editors for Haskell
| Mathew Mills wrote:
| > With Haskell's lovely strong static typing, it is a crying shame we
| > don't have an editor with immediate feedback, ala Eclipse.
| I've started writing an editor for Haskell. (It will be a commercial
| product)
| The first prototype was in C - now I'm re-writing from scratch in
| It is quite a tall order to provide immediate typed feedback of an
| buffer that will in general be syntactically incomplete but this is my
| eventual aim.
| One issue in the area of immediate feedback is that Haskell's syntax
| troublesome in a few areas. Consider:
|          foo :: SomeClass a => a -> a
| when the user has just typed:
|         foo :: SomeClass a
| should the editor assume SomeClass is a Tycon or a class name?
| One idea I had to solve this problem was to change the syntax of
| slightly so that constraints would be enclosed in {} instead of
| => ie:
|        foo :: {SomeClass a} a->a
| so that in
|       foo :: {SomeClass
| it is already determined that SomeClass must be a class name.
| Another thing which causes difficulty is the use of qualified
operators, and
| the fact that the qualification syntax is in the context free grammar
| instead of being kept in the lexical syntax (where I think it
belongs). For
| example, afaiu according to H98 (but not GHCi) it is permissible to
|            a Prelude      .     + b
|                 -- qvarsym  ->  [ modid . ] varsym
| whereas in my prototype I put all this into a level immediately above
| lexer but below the CFG so that no spaces are allowed thus:
|           a Prelude.+ b        -- no spaces in the qvarsym
|           a `Prelude.(+)` b    -- a little generalization
|           a `Prelude.(+) b        -- no need for closing `
| (The generalization above is intended for when you don't know whether
or not
| the function you're qualifying has been declared as an operator but
you want
| to use it as an operator eg if a pop-up list would appear after you
| `Prelude. with entries such as (+) plus add etc)
| With the above changes, it is possible to parse Haskell (or at least
as much
| as I got round to implementing in my C++ prototype) using a simple
| deterministic recursive descent parser with only 1 token of lookahead.
| (There is possibly some confusion in the H98 report about exactly how
| ambiguous expressions involving typed case alternatives might be
parsed eg x
| :: a->b -> if x 4 then ... but I'm hoping it will be ok to just fix
| syntax here by requiring extra brackets)
| Anyway I suppose the point of this post is to see whether or not
people feel
| that such changes are acceptable in an editor, or whether an editor
| adhere exactly to the standard (or whether the standard can be changed
| enable the determinism and ease of parsing necessary for interactive
| with immediate feedback)?
| Regards, Brian.
| --
| Logic empowers us and Love gives us purpose.
| Yet still phantoms restless for eras long past,
| congealed in the present in unthought forms,
| strive mightily unseen to destroy us.
| http://www.metamilk.com
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