Comments on current TypeHoles implementation

[Simon Peyton-Jones]

There is also the small matter, in this example, of distinguishing which `_' is which. The description works, but you have to think about it. I don't have an immediate and simple solution to this. Perhaps the addition of unique labels (e.g. _$1 _$2). But this is not a major problem. It can even wait until some future development/expansion on TypeHoles.

I have a proposal.  Someone has already suggested on hackage.haskell.org/trac/ghc/ticket/5910 that an un-bound variable behaves like a hole.  Thus, if you say

          f x = y
GHC says "Error: y is not in scope".  But (idea) with -XTypeHoles

f x = y
might generate

1.   (renamer) Warning: y is not in scope

2.   (type) Error: Hole "y" has type....
So that's like a named hole, in effect.

If you say
   f x = 4
GHC warns about the unused binding for x.  But if you say
   f _x = 4
the unused-binding warning is suppressed.  So (idea) if you say
          f x = _y
maybe we can suppress warning (1).  And, voila, named holes.

Moreover if you add -fdefer-type-errors you can keep going and run the program.

Any comments?  This is pretty easy to do.

(I'm unhappy that -XTypeHoles is a language pragma while -fdefer-type-errors is a compiler flag.  Maybe we should have -XDeferTypeErrors?)

Simon



From: sean.leather at gmail.com [mailto:sean.leather at gmail.com] On Behalf Of Sean Leather
Sent: 03 October 2012 16:45
To: Simon Peyton-Jones
Cc: GHC Users List; Thijs Alkemade
Subject: Comments on current TypeHoles implementation

Hi Simon,

Thanks for all your work in getting TypeHoles into HEAD. We really appreciate it.

I was playing around with HEAD today and wanted to share a few observations.

(1) One of the ideas we had was that a hole `_' would be like `undefined' but with information about the type and bindings. But in the current version, there doesn't appear to be that connection. This mainly applies to ambiguous type variables.

Consider:
> f = show _
The hole has type a0.

But with
> f = show undefined
there is a type error because a0 is ambiguous.

We were thinking that it would be better to report the ambiguous type variable first, rather than the hole. In that case, tou can use -fdefer-type-errors to defer the error. Currently, you don't have that option. I can see the argument either way, however, and I'm not sure which is better.

(2) There is a strange case where an error is not reported for a missing type class instance, even though there is no (apparent) relation between the missing instance and the hole. (This also relates to the connection to `undefined', but less directly.)

We have the following declaration:
> data T = T Int {- no Show instance -}

With a hole in the field
> g = show (T _)
we get a message that the hole has type Int.

With
> g = show (T undefined)
we get an error for the missing instance of `Show T'.

(3) In GHCi, I see that the type of the hole now defaults. This is not necessarily bad, though it's maybe not as useful as it could be.

ghci> :t show _
reports that the hole has type ().

(4) In GHCi, sometimes a hole throws an exception, and sometimes it does not.

ghci> show _
throws an exception with the hole warning message

ghci> show (T _)
and
ghci> _ + 42
cause GHCi to panic.

(5) There are some places where unnecessary parentheses are used when pretty-printing the code:

ghci> :t _ _

<interactive>:1:1: Warning:
    Found hole `_' with type t0 -> t
    Where: `t0' is a free type variable
           `t' is a rigid type variable bound by
               the inferred type of it :: t at Top level
    In the expression: _
    In the expression: _ (_)

<interactive>:1:3: Warning:
    Found hole `_' with type t0
    Where: `t0' is a free type variable
    In the first argument of `_', namely `_'
    In the expression: _ (_)
_ _ :: t

The argument `_' does not need to be printed as `(_)'.

There is also the small matter, in this example, of distinguishing which `_' is which. The description works, but you have to think about it. I don't have an immediate and simple solution to this. Perhaps the addition of unique labels (e.g. _$1 _$2). But this is not a major problem. It can even wait until some future development/expansion on TypeHoles.

Regards,
Sean
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