BSD-3-Clause licensed by Chris Done
Maintained by [email protected]
This version can be pinned in stack with:present-4.1.0@sha256:c52c9b80baa2c33311ffdf1cc070f3b64730920711c6b82df43acfbcb22ac469,733

Module documentation for 4.1.0


Make presentations for data types.


Requires: GHC 7.10.3

Install globally into your GHC’s global database (via stack) with:

$ sh

To remove it later:

$ sh

To be added: install via stack install present, but it needs to be added to an LTS/nightly version first.

Customizing GHCi

Add to the following to your ~/.ghci:

:seti -XTemplateHaskell
:def presentDebug \e -> return ("let it = " ++ e ++ "\n$(Present.presentIt)")
:def presentQualified \e -> return ("let it = " ++ e ++ "\nPrelude.putStrLn (Present.toShow True $(Present.presentIt))")
:def present \e -> return ("let it = " ++ e ++ "\nPrelude.putStrLn (Present.toShow False $(Present.presentIt))")
:set -package present


bash-3.2$ stack exec ghci
GHCi, version 7.10.3:  :? for help
package flags have changed, resetting and loading new packages...
λ> :present "hi"

λ> :present 123

λ> :present (5.2,"hi",1,'a')

λ> data X a = X a Int

λ> :present X 5 1
X 5 1

λ> :present S88.pack "hi"

λ> :present print
<a_0 -> GHC.Types.IO ()>

λ> :present print ()
IO (<GHC.Prim.State# GHC.Prim.RealWorld -> (GHC.Prim.State# GHC.Prim.RealWorld, a_0)>)

λ> :present undefined
<_ :: t_0>

λ> :present id
<a_0 -> a_0>

λ> :present [undefined]
[<_ :: t_0>]


You can write your own instances like this:

λ> data X = X Int
λ> :present X 5
X 5
λ> instance Present0 X where present0 = ("X",\(X x) -> IntegerPresentation "X" (show x))
λ> :present X 5

Detailed output

λ> :presentDebug (2.5,"hi",1,'a')
     [("Floating",IntegerPresentation "GHC.Types.Double" "2.5")
     ,("Show",IntegerPresentation "GHC.Types.Double" "2.5")
     ,("Rational",IntegerPresentation "GHC.Types.Double" "5/2")
      ,DataTypePresentation "GHC.Types.Double"
                            [PrimitivePresentation "GHC.Prim.Double#"])]
     [("String",StringPresentation "String" "hi")
     ,("List of characters"
            [("Character",CharPresentation "GHC.Types.Char" "h")
            ,("Unicode point"
                [("Decimal",IntegerPresentation "GHC.Types.Char" "104")
                ,("Hexadecimal",IntegerPresentation "GHC.Types.Char" "68")
                ,("Binary",IntegerPresentation "GHC.Types.Char" "1101000")])
             ,DataTypePresentation "GHC.Types.Char"
                                   [PrimitivePresentation "GHC.Prim.Char#"])]
            [("Character",CharPresentation "GHC.Types.Char" "i")
            ,("Unicode point"
                [("Decimal",IntegerPresentation "GHC.Types.Char" "105")
                ,("Hexadecimal",IntegerPresentation "GHC.Types.Char" "69")
                ,("Binary",IntegerPresentation "GHC.Types.Char" "1101001")])
             ,DataTypePresentation "GHC.Types.Char"
                                   [PrimitivePresentation "GHC.Prim.Char#"])]])]
     [("Decimal",IntegerPresentation "GHC.Integer.Type.Integer" "1")
     ,("Hexadecimal",IntegerPresentation "GHC.Integer.Type.Integer" "1")
     ,("Binary",IntegerPresentation "GHC.Integer.Type.Integer" "1")
      ,DataTypePresentation "GHC.Integer.Type.Integer"
                            [PrimitivePresentation "GHC.Prim.Int#"])]
     [("Character",CharPresentation "GHC.Types.Char" "a")
     ,("Unicode point"
         [("Decimal",IntegerPresentation "GHC.Types.Char" "97")
         ,("Hexadecimal",IntegerPresentation "GHC.Types.Char" "61")
         ,("Binary",IntegerPresentation "GHC.Types.Char" "1100001")])
      ,DataTypePresentation "GHC.Types.Char"
                            [PrimitivePresentation "GHC.Prim.Char#"])]]