Polymorphic functions are characterised by type variables denoted by
and an optional index. Each time a polymorphic MAL function is called, the symbol table is first inspected for the matching strongly typed version. If it does not exists, a copy of the MAL program is generated, whereafter the type variables are replaced with their concrete types. The new MAL program is immediately type checked and, if no errors occured, added to the symbol table.
The generic type variable :any designates an unknown type, which may be filled at type resolution time. Unlike indexed polymorphic type arguments, :any type arguments match possibly with different concrete types.
An example of a parameterised function is shown below:
function user.helloWorld(msg:any_1):any_1; io.print(msg); return user.helloWorld; end helloWorld;
The type variables ensure that the return type equals the argument type. Type variables can be used at any place where a type name is permitted. Beware that polymorphic typed variables are propagated throughout the function body. This may invalidate type resolutions decisions taken earlier (See MAL Type System).
This version of helloWorld can also be used for other arguments types, i.e. bit,sht,lng,flt,dbl,.... For example, calling helloWorld(3.14:flt) echoes a float value.