HSFFIG.FieldAccess

Documentation

class FieldAccess a b c | a c -> b where

A multi-parameter class with functional dependencies is declared to enable access to structures/unions members. The functional dependencies a b c | a c -> b specify that type of b (type of the member) depends entirely on the types a (phantom type identifying structure/union) and c (phantom type identifying the member, a.k.a. member selector), and there may be only one type of b for every possible combination of a and c.

Indeed:

 struct a {
   int x;
 };
 struct b {
   float x;
 };

both structures contain a member with the same name, but different types.

HSFFIG will generate the following instances for the structures above:

 instance HSFFIG.FieldAccess.FieldAccess S_a ((CInt)) V_x where
   z --> V_x = (#peek __quote__(struct a), x) z
   (z, V_x) <-- v = (#poke __quote__(struct a), x) z v
 ...
 instance HSFFIG.FieldAccess.FieldAccess S_b ((CFloat)) V_x where
  z --> V_x = (#peek __quote__(struct b), x) z
  (z, V_x) <-- v = (#poke __quote__(struct b), x) z v

That is, when the member identified by selector V_x is fetched from struct a (S_a), an integer value is returned. But when the member with the same name is retrieved from struct b, a float value is returned.

Methods

(==>)

Arguments

:: Ptr a

-> c

-> b

retrieves a function reference from a member of structure a with name selector c of type b which in this case is a function type (thus ==> is called in non-monadic context). Selector for c may only be X_-prefixed.

(-->)

Arguments

:: Ptr a

-> c

-> IO b

retrieves a value (mutable, thus monadic context is required) from a member of structure a with name selector c of type b. Selector for c may only be V_-prefixed.

(<--)

Arguments

:: (Ptr a, c)

-> b

-> IO ()

destructively updates a member of structure a with name selector c with value of type b. Selector for c may only be V_-prefixed.