Enabling TypeHoles by default

[Mateusz Kowalczyk]

On 13/01/14 18:42, Krzysztof Gogolewski wrote:
> Hello,
> 
> As discussed on ghc-devs, I propose to enable -XTypeHoles in GHC by
> default. Rationale:
> 
> (1) This way holes are far easier to use; just entering "_" allows to check
> type of a subexpression, no need of adding "-XTypeHoles".
> 
> (2) This affects error messages only; i.e. the set of programs that compile
> stays the same - Haskell 2010. The only exception is that if you use
> -fdefer-type-errors, then a program with a hole compiles, but this seems
> fine with philosophy of -fdefer-type-errors.
> 
> If so: would you like it to be in 7.8 or wait a cycle? My preference is
> 7.8, two people (John and Richard) suggested 7.10.
> 
> -KG
> 

Sounds good. Are there plans to allow TypeHoles to actually sit in place
of types? In the past I did

```
data Hole

hole :: Hole
hole = undefined

foo :: Integer -> Integer
foo x = hole

bar :: Integer -> Hole
bar x y = x + y
```

to cause type errors that could guide me. I now don't have to resort to
the first use in ‘foo’ but I still have to define my own Hole type in ‘bar’.


-- 
Mateusz K.