Add Float.pi

Fixes https://caml.inria.fr/mantis/view.php?id=4170

Changes

@@ -65,6 +65,9 @@ Working version
 - GPR#1638: add Float module.
   (Nicolás Ojeda Bär, review by Alain Frisch and Jeremy Yallop)
 
+- GPR#1674, MPR#4170: add the constant `Float.pi`.
+  (Christophe Troestler, review by Damien Doligez)
+
 ### Other libraries:
 
 - MPR#7745, GPR#1629: Graphics.open_graph displays the correct window title on

stdlib/float.ml

@@ -25,6 +25,7 @@ external abs : float -> float = "%absfloat"
 let infinity = Pervasives.infinity
 let neg_infinity = Pervasives.neg_infinity
 let nan = Pervasives.nan
+let pi = 0x1.921fb54442d18p+1
 let max_float = Pervasives.max_float
 let min_float = Pervasives.min_float
 let epsilon = Pervasives.epsilon_float

stdlib/float.mli

@@ -66,6 +66,9 @@ val nan : float
     [=], [<], [<=], [>] and [>=] return [false] and [<>] returns [true]
     if one or both of their arguments is [nan]. *)
 
+val pi : float
+(** The constant pi. *)
+
 val max_float : float
 (** The largest positive finite value of type [float]. *)