Would it be possible to have no command at all? Types are
distinguished by upper-case letters, so it should be possible to tell
whether a given expression is a value-level or a type-level expression.
I guess that's not strictly true, since the expression could be _only_
type variables -- but then I think it would be forgivable to just use
the value-level evaluator for those ambiguous ones.
~d
Quoting Simon Peyton-Jones
Sean
Yes, this has been asked for before, and it wouldn't be hard to implement.
What should the GHCi command be *called*?
We already have :kind, which displays the kind of a type. Maybe :kind! should evaluate the type as well? Or perhaps :kind should evaluate anyway (although that would be a bit inconsistent with :type which does not evaluate the expression)
Or :normtype? short for normalise type
Simon
From: glasgow-haskell-users-bounces@haskell.org [mailto:glasgow-haskell-users-bounces@haskell.org] On Behalf Of Sean Leather Sent: 20 September 2011 11:34 To: GHC Users List Subject: Evaluating type expressions in GHCi
I would like to ask GHCi for the type that a type expression will evaluate to, once all definitions of type synonyms and (when possible) type families have been inlined.
It appears that I can do some part of this for type T by using ":t undefined :: T":
type family F a type instance F Int = Bool type instance F Bool = Int type instance F (a, b) = (F a, F b)
ghci> :t undefined :: F (Int, Bool) undefined :: F (Int, Bool) :: (Bool, Int)
I also get what I expect here:
ghci> :t undefined :: F (a, Bool) undefined :: F (a, Bool) :: (F a, Int)
Of course, this doesn't work on types of kinds other than *.
Is it possible and worth having another GHCi command to perform this operation for any types? It could be the type analogue to :t such that it evaluates the type and gives its kind.
Regards, Sean