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Mal – Make a Lisp, in 68 languages

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Re: Mal – Make a Lisp, in 68 languages

#7

Post by gbacon »

yosyp wrote: post #4

Could someone explain this gem in the C version? https://github.com/kanaka/mal/blob/master/c/types.c#L170

In struct MalVal[0], the values f0 through f20, where each holds a pointer-to-functions of the corresponding arity, live in a big (multifacted?) union. Using arg_cnt, the switch assigns func to the appropriate value in mv->val using the appropriate cast.

For example, when arg_cnt is 2, mv->val.f2 receives func converted to void * (* )(void* , void* ), that is, pointer to a function that accepts two parameters of type pointer-to-void and returns pointer-to-void. [Weird spacing to effectively escape the asterisks that would otherwise be treated as italicizing markup.]

C permits conversion between pointer-to-function of one type to pointer-to-function of another type. The inverse is also defined, and the resulting pointer will compare equal to the original. However, calling a function through a pointer-to-function where the types do not match invokes undefined behavior.

[0]: https://github.com/kanaka/mal/blob/master/c/types.h#L85

Re: Mal – Make a Lisp, in 68 languages

#9

Post by copx »

gbacon wrote: post #7
yosyp wrote: post #4

Could someone explain this gem in the C version? https://github.com/kanaka/mal/blob/master/c/types.c#L170

In struct MalVal[0], the values f0 through f20, where each holds a pointer-to-functions of the corresponding arity, live in a big (multifacted?) union. Using arg_cnt, the switch assigns func to the appropriate value in mv->val using the appropriate cast. For example, when arg_cnt is 2, mv->val.f2 receives func converted to void * (* )(void* , void* ), that is, pointer to a function that accepts two parameters of type…

This could be made massively less ugly and more readable by typedef'ing the function pointer types by the way. Then one could just write:

  case 13: mv->val.f13 = (F13)func; break;
As a C programmer I got to say people in general don't use typedef for function pointer types often enough. The types are so ugly and verbose, they should be typedef'ed by default.
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