I was looking at some code, saw a variable x, and said to myself, "Ah that variable is a monad." Then I realized "Monad" is the name of a type class. So maybe x should be called "an instance of a Monad." I think the word "instance" in this case is OO-like; but in Haskell "instance" refers to a type that is an instance of a type class. Or maybe it can refer to both? And Monad is a type class, not a type. Maybe I need the phrase "monadic type" to refer to an instance of a type class. So maybe x is just "a variable of a monadic type"? Thanks, Mike
On Aug 12, 2009, at 21:59 , Michael P Mossey wrote:
I was looking at some code, saw a variable x, and said to myself, "Ah that variable is a monad." Then I realized "Monad" is the name of a type class. So maybe x should be called "an instance of a Monad." I think the word "instance" in this case is OO-like; but in Haskell "instance" refers to a type that is an instance of a type class. Or maybe it can refer to both? And Monad is a type class, not a type. Maybe I need the phrase "monadic type" to refer to an instance of a type class. So maybe x is just "a variable of a monadic type"?
Strictly speaking, yes. In practice the common shorthand is "in the X monad" or just "in X". -- brandon s. allbery [solaris,freebsd,perl,pugs,haskell] allbery@kf8nh.com system administrator [openafs,heimdal,too many hats] allbery@ece.cmu.edu electrical and computer engineering, carnegie mellon university KF8NH
Michael P Mossey wrote:
I was looking at some code, saw a variable x, and said to myself, "Ah that variable is a monad." Then I realized "Monad" is the name of a type class. So maybe x should be called "an instance of a Monad." I think the word "instance" in this case is OO-like; but in Haskell "instance" refers to a type that is an instance of a type class. Or maybe it can refer to both? And Monad is a type class, not a type. Maybe I need the phrase "monadic type" to refer to an instance of a type class. So maybe x is just "a variable of a monadic type"?
One common nomenclature is to say that x is a "monadic action" or just "action" for short. That's the terminology used in the Simon Peyton Jones. Tackling the awkward squad. http://research.microsoft.decenturl.com/awkward-squad tutorial. Regards, apfelmus -- http://apfelmus.nfshost.com
I was looking at some code, saw a variable x, and said to myself, "Ah that variable is a monad." Then I realized "Monad" is the name of a type class. So maybe x should be called "an instance of a Monad." I think the word "instance" in this case is OO-like; but in Haskell "instance" refers to a type that is an instance of a type class. [...]
In general, it helps to know that the current "de facto" Haskell standard actually accepts classes and instances built on top of many types: class SomeClass a b c where someF :: a -> b -> c anotherF :: b -> c -> a instance SomeClass T1 T2 T3 where someF = ... anotherF = ... This way it becomes clear that saying that some type "is of" or "instantiate" some class isn't apropriate. Best, Maurício
participants (4)
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Brandon S. Allbery KF8NH -
Heinrich Apfelmus -
Maurício CA -
Michael P Mossey