Comments on current TypeHoles implementation

[Sean Leather]

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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