Type

Posted on 2017-11-03 by xkollar in Bash, Haskell, Fun.

Have you ever semi-automatically quasi-subconsciously run :t outside GHCi?

$ type :t
:t is a function
:t () 
{ 
    ghci -v0 <<< ":t ${*:-undefined}"
}

What it can do:

$ :t map
map :: (a -> b) -> [a] -> [b]

$ :t fold

<interactive>:1:1: error:
    • Variable not in scope: fold
    • Perhaps you meant one of these:
        ‘foldr’ (imported from Prelude), ‘foldl’ (imported from Prelude)

Whops… what about

$ :t Data.Foldable.fold
Data.Foldable.fold :: (Monoid m, Foldable t) => t m -> m

That is better. Maybe we can poke also other packages, not just base…

$ :t Data.Time.addUTCTime
Data.Time.addUTCTime
  :: time-1.6.0.1:Data.Time.Clock.UTC.NominalDiffTime
     -> time-1.6.0.1:Data.Time.Clock.UTC.UTCTime
     -> time-1.6.0.1:Data.Time.Clock.UTC.UTCTime

Awesome :-).

In the next episode we will deal with accidental runs of ls in GHCi.