[Haskell-cafe] Applicative Functor or Idiom?

Twan van Laarhoven twanvl at gmail.com
Fri Nov 20 14:58:24 EST 2009

David Sankel wrote:
> After reading several recent papers I came to the understanding that
> there isn't consensus on the name of Applicative Functors. Several
> prefer to call them idioms:
> "'Idiom' was the name McBride originally chose, but he and Paterson
> now favour the less evocative term `applicative functor'. We have a
> slight preference for the former, not least because it lends itself
> nicely to adjectival uses, as in `idiomatic traversal'"[1]
> I also noticed use of the term Idiom in [2], [3], and [4].

I much prefer the name Applicative Functor, because 'idiom' and especially 
'idiomatic' can mean lots of other things (just look up the word in a 
dictionary!). While an applicative functor is always a functor with 'pure' and 
'ap' operations.

 > I'm writing a set of classes that includes AF's and I'm trying to
 > decide whether to call the class Idiom. Anyone have more information
 > on this question?

Why are you writing your own? How do your classes differ from the standard 


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