Brian Hulley wrote:
When you try to write an editor for Haskell (or some subset of it), you
quickly discover these areas of Haskell syntax like the above which need
to be changed to get an optimum interactive editing experience. I think
it *is* possible to adjust the Haskell grammar so that it is LL(1) and
the only reason it is not already LL(1) seems to be that the grammar has
been designed with compilers (which only need to deal with complete
modules) in mind rather than programmers interactively editing in mind.
(The other change needed for LL(1) is to give contexts a marker before
they appear eg:
foo :: {MonadIO m} a -> m a
)
Just catching up on haskell-cafe...
The other notorious part of the Haskell grammar that isn't LL/LR(1) is
expressions vs. patterns. In a statement, if you see a variable, you don't know
whether it is a pattern variable (apat) or an expression variable (aexpr). This
is why Haskell grammars generally parse expressions and patterns using the same
non-terminals.
By LL(1) I'm really meaning that the grammar for interactive editing
needs to be adjusted so that it is possible to maintain the invariant
that as code is entered from left to right constructs and identifiers
can be highlighted according to their grammatical role and highlighting
(modulo incompleteness) must remain unchanged regardless of whatever is
typed afterwards to the right otherwise it can become more of a
liability than a help, hence my hope that some future revision of
Haskell grammar might consider taking the above points into account.
So you won't be able to colour patterns differently from expressions, that
doesn't seem any worse than the context vs. type issue. Indeed, I'm not even
sure you can colour types vs. values properly, look at this:
data T = C [Int]
at this point, is C a constructor? What if I continue the declaration like
this:
data T = C [Int] `F`
now it's a type!
Cheers,
Simon
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