On Tue, Sep 11, 2012 at 12:59 PM, Stayvoid wrote:

ghci> Map.fromList [("betty","555-2938"),("bonnie","452-2928"),("lucille","205-2928")] fromList [("betty","555-2938"),("bonnie","452-2928"),("lucille","205-2928")]

Why does Map.fromList represent its output like this?

It doesn't; the Show instance for Data.Map, however, does a toAscList and then wraps the resulting list in a fromList, because the internals of a Map are deliberately hidden. If the internals of Map were visible, it would be possible to build an invalid Map; at best it would be unbalanced and slow, at worst it would be misordered and keys would randomly go "missing" as a result. See, for example, how (Map Double a) misbehaves (try searching the haskell-cafe list for "Map Double"): the poorly defined Ord constraint on floating point types, especially in the presence of NaN, means keys *do* sometimes go missing in this case and (Map Double a) ends up being rather unreliable. This also means you can break a Map by providing an incorrect or inconsistent Ord instance for its key. This is why typeclass instances are global; if there were local instances, you could do severe violence to a Map by providing an incompatible/incoherent local Ord instance for the type of its key. data Map.Map k a

= Data.Map.Tip | Data.Map.Bin !Data.Map.Size !k a !(Map.Map k a) !(Map.Map k a)

A node in a Map is either a Tip (placeholder of sorts) or a Bin comprising a strict number of subnodes/values, a strict key and its associated value, and strict left and right subtrees (which may be Tip-s if this is a leaf). You should never try to work with this directly (and under normal circumstances you can't, as the constructors are hidden), as outlined above. -- brandon s allbery allbery.b@gmail.com wandering unix systems administrator (available) (412) 475-9364 vm/sms