[Haskell-cafe] Functions from Data.Hashtable

Thomas Davie tom.davie at gmail.com
Wed Apr 8 02:55:26 EDT 2009

On 8 Apr 2009, at 00:46, wren ng thornton wrote:

> Peter Verswyvelen wrote:
>> These functions have their arguments reversed when compare to e.g.  
>> Map
>> For example
>> Data.HashTable.lookup :: HashTable key val -> key -> IO (Maybe val)
>> Data.Map.lookup :: Ord key => key -> Map key val -> Maybe val
>> I find this a bit odd. Is this by design?
> As a design issue, I think the dominant order is better for partial  
> application since they make good arguments to  
> Control.Monad.State.gets and similar state-like HOFs. Considering  
> the IO ugliness of HashTable, Jason Dagit may have a point though.  
> As I recall, this discussion has lead to flame wars in the past.

To add my 2¢ to this one – The arguments for each one can be put  
across as this:

  • I prefer (M a b) -> a -> b, because we can do functional  
programming in it more easily.
  • I prefer a -> (M a b) -> b, because we can do stateful programming  
in it more easily.

Perhaps we need to remember what kind of language Haskell is – yes  
it's an excellent imperative language, but when it gets down to it,  
it's functional, and maybe we should try to keep it that way.

To add support to the other argument though – I think the stateful  
version reads better when used fully applied – it reads as lookup this  
key in this map.  The reverse doesn't read so well in english, the  
best I can think of is, transform this map into a function and apply  
it, but that doesn't actually include the word lookup anywhere.


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