Re: [ats-lang-users] template functions vs polymorpdic functions

[Hongwei Xi <hw...@cs...>]

On Mon, 5 Apr 2010, Rouan van Dalen wrote:

>>Hi Hongwei
>>
>>I was wondering why ATS has template functions AND polymorphic functions?
>>
>>Where would I rather use a template than a polymorphic function?

Say you have the following function:

fun f {a:type} (x: a): a = x

'f' is polymorphic. However, 'f' can only be applied to a boxed value. 
E.g., f(0.0) results in a type error as 'double' is not a boxed type. BTW, 
all datatypes are boxed.

Suppose we implement f as follows:

fun f {a:t@ype} (x: a): a = x

Unfortuately, this function cannot be compiled as the size of 'a' is 
unknown at compile-time. In ATS, 'f' can be implemented as follows:

fun{a:t@ype} f (x: a): a = x

This is a function template. Just like in C++, a template is not
compiled until it is applied. If you call f(1), then *essentially*,
the following function is generated

fun f_int (x: int): x = x

and the call becomes f_int(1). If you call f(1.0), then the following
function is generated:

fun f_double (x: int): x = x

and the call becomes f_double(1.0).

In pratice, I find templates to be extremely useful, especially, when
implementing library code.

Hope this helps,

--Hongwei

Computer Science Department
Boston University
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