List: forEach: arg1;

[glen e. p. ropella]

Ludo Pagie writes:
 > The compiler only complains about the line
 > [metabList forEach: M(addConcentration:) : myarg];
 > with:
 > 
 > >reaction.m: In function `_i_Reaction__react_':
 > >reaction.m:38: incompatible type for argument 4 of indirect function call

This is just a shot in the dark.... But, the "forEach::" method
is specified as:

   - (void) forEach: (SEL)aSelector : arg1

which can be read as

   - (void) forEach: (SEL)aSelector : (id) arg1

However, you're arg1 is of type (double) instead of (id).  Now,
why it says there's a problem with "argument 4" I have no idea...
unless the parsing of [object message: arg : arg] works out 
such that "message:" is argument 1, "arg" is argument 2, ":" is
argument 3, and "arg" is argument 4.

But, anyway, try replacing 

   [metabList forEach: M(addConcentration:) : myarg];

with

   [metabList forEach: M(addConcentration:) : (id) myarg];

and see if that helps.  

glen
p.s. Thanks for including code in the message!