Enabling TypeHoles by default

Mateusz Kowalczyk fuuzetsu at fuuzetsu.co.uk
Mon Jan 13 20:54:44 UTC 2014

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.

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