Record-type recursive member functions and the “rec” keyword


Record-type recursive member functions and the “rec” keyword



I've always believed that in F# we needed to use the rec keyword for every recursive function, for example:

let rec factorial = function | 0 -> 1 | k when k > 0 ->  k * (factorial (k - 1)) | failwith "oops!" 

Today I was playing around with F# and I came up with a code similar to the following:

let MyRecordType =     { Something     : float;       SomethingElse : int }     with         static member factorial = function             | 0 -> 1             | k when k > 0 ->  k * (MyRecordType.factorial (k - 1))             | failwith "oops!" 

As you see, I've just defined a recursive function, but I made what at first seemed like a mistake: I forgot to declare the function as recursive by means of the rec keyword.

But to my surprise it compiles! And there's more to it: if you add the rec keyword, then it is a syntax error!

type MyRecordType =     { (* ... *) }     with         // syntax error:         static member rec factorial = function         (* ... *) 

I've googled around for an explanation but got nothing. In the MSDN documentation, I couldn't find any mention to the rec keyword outside the page about recursive functions, and as of 2010-01-03 it does not mention the case I'm asking about.

Exactly the same thing happens with non-static members.

So, why is it a syntax error to use the rec keyword on member functions of a record-type?




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'let rec' isn't about defining recursive functions, but defining a binding in an environment, that includes the binding for the current variable to be bound.


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You could use 'let rec' just as well to define e.g.


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Often, you don't want the binding to be included in the environment, as you might want to access an earlier variable by the same name..
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When you are defining the static member function, factorial, you aren't looking for a binding for a variable 'factorial', but for a type 'MyRecordType' (which is in the environment as a type definition), and if it happens to have a static member function called 'factorial', which it has..
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All "member" functions are implicitly "rec" within the type they're defined in..



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