Am Donnerstag, 23. Oktober 2008 05:36 schrieb Larry Evans:
Thanks Deaniel. The fog in my head begins to clear. I took Antoine's suggestion and got: <---cut here --- GHCi, version 6.8.2: http://www.haskell.org/ghc/ :? for help Loading package base ... linking ... done. Prelude> :load "/home/evansl/prog_dev/haskell/my-code/uniplate.try.phantom.hs" [1 of 1] Compiling Main ( /home/evansl/prog_dev/haskell/my-code/uniplate.try.phantom.hs, interpreted ) Ok, modules loaded: Main. *Main> let val_9 = Val 9 Loading package mtl-1.1.0.0 ... linking ... done. Loading package array-0.1.0.0 ... linking ... done. Loading package containers-0.1.0.1 ... linking ... done. Loading package uniplate-1.2.0.1 ... linking ... done. *Main> :t val_9 val_9 :: Expr e *Main> print val_9 Val 9 *Main>
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I guess the phantom type mentioned in Antoine's post is the e in:
val_9::Expr e
?
Exactly. That type is never used in any value of type (Expr e), therefore it is called a 'phantom' type. It only serves as a tag to prevent mixing Expr's with different type parameters. Cheers, Daniel