Unboxed type

Unboxed types are types that represent raw values. Unboxed types have kind #.

Note that unboxed types of different storage behaviours (four bytes, eight bytes etc.) are all lumped together under kind #. As a result, type variables must have kinds which are #-free.

From the old Wiki

One might imagine numbers naively represented in Haskell "as pointer to a heap-allocated object which is either an unevaluated closure or is a "box" containing the number's actual value, which has now overwritten the closure" [3. below]. GHC (and other implementations?) allow direct access to the value inside the box, or "Unboxed" values.

In GHC, by convention(?), unboxed values have a hash mark as a suffix to their name. For instance, the unboxed reprsentation of 42 is 42#. There are some restrictions to their use. In particular, you can't pass them to polymorphic functions (like show for instance).

In this example, I# is a constructor that takes an unboxed integer and returns the Int that we know and love.

module Main where
import GHC.Exts

showUnboxedInt :: Int# -> String
showUnboxedInt n = (show $ I# n) ++ "#"

Here we wrap the unboxed Int n with the I# constructor and show the regular-old Int, whith a hash mark on the end.

Unboxed Tuples and Arrays

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When to use Unboxed Types

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See Also

1. See the discussion on primitives in the GHC's User's Guide.
2. See the GHC.Exts module.
3. See SPJ's paper Unboxed values as first class citizens.

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This page is a work in progress by IsaacJones. More input welcome :)