Evaluating type expressions in GHCi
simonpj at microsoft.com
Wed Sep 21 00:44:18 CEST 2011
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
From: glasgow-haskell-users-bounces at haskell.org [mailto:glasgow-haskell-users-bounces at 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.
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