Evaluating type expressions in GHCi

Simon Peyton-Jones 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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