Merge pull request #1729 from ethereum/constantvariables

Only allow pure expressions for constant state variables.

1 files changed, 20 insertions, 6 deletions

diff --git a/docs/contracts.rst b/docs/contracts.rst
index 1516ba0b..9145f016 100644
--- a/docs/contracts.rst
+++ b/docs/contracts.rst
@@ -428,8 +428,25 @@ change by overriding).
 Constant State Variables
 ************************
 
-State variables can be declared as constant (this is not yet implemented
-for array and struct types and not possible for mapping types).
+State variables can be declared as ``constant``. In this case, they have to be
+assigned from an expression which is a constant at compile time. Any expression
+that accesses storage, blockchain data (e.g. ``now``, ``this.balance`` or
+``block.number``) or
+execution data (``msg.gas``) or make calls to external contracts are disallowed. Expressions
+that might have a side-effect on memory allocation are allowed, but those that
+might have a side-effect on other memory objects are not. The built-in functions
+``keccak256``, ``sha256``, ``ripemd160``, ``ecrecover``, ``addmod`` and ``mulmod``
+are allowed (ever though they do call external contracts).
+
+The reason behind allowing side-effects on the memory allocator is that it
+should be possible to construct complex objects like e.g. lookup-tables.
+This feature is not yet fully usable.
+
+The compiler does not reserve a storage slot for these variables and every occurrence is
+replaced by the respective constant expression (which might be computed to a single value by the optimizer).
+
+Not all types for constants are implemented at this time. The only supported types are
+value types and strings.
 
 ::
 
@@ -438,12 +455,9 @@ for array and struct types and not possible for mapping types).
     contract C {
         uint constant x = 32**22 + 8;
         string constant text = "abc";
+        bytes32 constant myHash = keccak256("abc");
     }
 
-This has the effect that the compiler does not reserve a storage slot
-for these variables and every occurrence is replaced by their constant value.
-
-The value expression can only contain integer arithmetics.
 
 ******************
 Constant Functions