Re: transpiling to low level C

On 2024-12-21, Janis Papanagnou <janis_papanagnou+ng@hotmail.com> wrote:
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On 21.12.2024 02:28, Tim Rentsch wrote:
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Janis Papanagnou <janis_papanagnou+ng@hotmail.com> writes:
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On 16.12.2024 00:53, BGB wrote:
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[...]
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Pretty much all higher level control flow can be expressed via
goto.

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A 'goto' may be used but it isn't strictly *necessary*. What *is*
necessary, though, that is an 'if' (some conditional branch), and
either 'goto' or recursive functions.

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Conditional branches, including 'if', '?:', etc., are not strictly
necessary either.

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No? - Can you give an example of your statement?

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In a functional langauge, we can make a decision by, for instance,
putting two lambdas into an array A, and then calling A[0] or A[1],
where the index 0 or 1 is comes from some Boolean result.
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The only reason we have a control construct like if(A, X, Y) where X
is only evaluated if A is true, otherwise Y, is that X and Y
have side effects.
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If X and Y don't have side effects, then if(A, X, Y) can be an ordinary
function whose arguments are strictly evaluated.
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Moreover, if we give the functional language lazy evaluation
semantics, then anyway we get the behavior that Y is not evaluated
if A is true, and that lazy evaluation model can be used as the
basis for sneaking effects into the functional language and
conctrolling them.
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Anyway, Turing calculation by primitive recursion does not require
conditional branching.  Just perhaps an if function which returns
either its second or third argument based on the truth value of its
first argument.
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For instance, in certain C preprocessor tricks, conditional
expansion is achieved by such macros.
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When we run the following through the GNU C preprocessor (e.g. by
pasting into gcc -E -x c -p -):
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  #define TRUE_SELECT_TRUE(X) X
  #define TRUE_SELECT_FALSE(X)
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  #define FALSE_SELECT_TRUE(X)
  #define FALSE_SELECT_FALSE(X) X
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  #define SELECT_TRUE(X) X
  #define SELECT_FALSE(X)
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  #define PASTE(X, Y) X ## Y
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  #define IF(A, B, C) PASTE(TRUE_SELECT_, A)(B) PASTE(FALSE_SELECT_, A)(C)
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  #define FOO TRUE
  #define BAR FALSE
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  IF(FOO, foo is true, foo is false)
  IF(BAR, bar is true, bar is false)
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We get these tokens:
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  foo is true
   bar is false
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Yet, macro expansion has no conditionals.  The preprocessing
language has #if and #ifdef, but we didn't use those.  Just
expansion of computed names.