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1.3  Static Values

Static values are infinite constant streams made of a value and they are introduced with the construction let static. Static values are usefull to define parameterised systems. For example:

let static m = 100.0
let static g = 9.81
let static mg = m *. g

val mg :  float
val mg :: static

A static value is distinguished from the other by its clock: the clock static means that the value can be computed once for all at instantiation time, before the execution starts.

It is possible to impose that the input of a function be a static value. For example:

let node integr (static dt) x0 dx = x where
  rec x = x0 -> pre x +. dx *. dt

val integr :  float -> float => float
val integr :: static -> ’a -> ’a

The definition of a static value is valid if the right-hand part of the definition is a constant stream. In the present version of the compiler, a stream is said to be constant when it is both combinatorial and its clock can be fully generalized.

A static expression is thus not necessarily an immediate constant. It can be any combinatorial expression which only depend on other static expressions. This is why the following program is rejected:

let node wrong x0 dt = 
  integr (0.0 -> 1.0) x0 dt

File "", line 15, characters 10-20:
>  integr (0.0 -> 1.0) x0 dt
>          ^^^^^^^^^^
This expression has clock ’b,
but is used with clock static.

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