GHC.Types consturctors with #

[David Peixotto]

Hi Larry,

GHC allows you to work with unboxed types. Int# is the type of unboxed ints. I# is a normal data constructor. So we can see that GHC represents a (boxed) Int as a normal algebraic data type

data Int = I# Int#

which says that an Int is a type with a single constructor (I#) that wraps a machine integer (Int#). By convention, unboxed types use a # in their name.

You can find more info about unboxed types here: http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/primitives.html#glasgow-unboxed

To work with unboxed types in your code (or ghci) you need the MagicHash extension: http://www.haskell.org/ghc/docs/6.12.2/html/users_guide/syntax-extns.html#magic-hash

$ ghci -XMagicHash
GHCi, version 6.12.3: http://www.haskell.org/ghc/  :? for help
Loading package ghc-prim ... linking ... done.
Loading package integer-gmp ... linking ... done.
Loading package base ... linking ... done.
Loading package ffi-1.0 ... linking ... done.
Prelude> import GHC.Types
Prelude GHC.Types> I# 5#
5
Prelude GHC.Types> 

-David


On Nov 1, 2010, at 12:40 PM, Larry Evans wrote:

> http://www.haskell.org/ghc/docs/6.10.2/html/libraries/ghc-prim/GHC-Types.html
> 
> contains:
> 
> data Int = I# Int#
> 
> What does I# Int# mean?  I've tried a simple interpretation:
> 
>  Prelude GHC.Types> I# 5#
> 
>  <interactive>:1:5: parse error (possibly incorrect indentation)
>  Prelude GHC.Types>
> 
> but obviously that failed :(
> 
> TIA.
> 
> -Larry
> 
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