Core Erlang 的 `letrec` 有什么用？

Question

Core Erlang 的 letrec 有什么用？

Richard Carlsson 在 "Introduction to Core Erlang" 中写道：

Furthermore letrec expressions allow local (recursive) function definitions, which ERLANG itself does not have, but which are often useful in transformations.

letrec 对哪些转换有用？

从 Erlang 翻译时 erlc 是否实际生成 letrec？或者只有从非 Erlang 源语言翻译时才会生成 letrecs？

Answer 1

Nowadays, list comprehensions are translated to letrecs (which are available in Core Erlang), and the letrecs are in turn translated to ordinary recursive functions.

请注意，“现在”是 2010 年。另外，在 EEP 52: Allow key and size expressions in map and binary matching 中有一些使用示例：

In OTP 23, all variables used in a segment size expression must be already bound in the enclosing environment. The previous example must be rewritten like this using nested cases:

   'foo'/1 =
       fun (_0) ->
             case _0 of
                 <#{#<Sz>(16,1,'integer',['unsigned'|['big']]),
                  #<_2>('all',1,'binary',['unsigned'|['big']])}#> when 'true' ->
                     case _2 of
                        <#{#<X>(Sz,1,'integer',['unsigned'|['big']])}#> when 'true' ->
                            X
                        <_3> when 'true' ->
                            %% Raise function_clause exception.
                            .
                            .
                            .
                       end
                  <_4> when 'true' ->
                       %% Raise function_clause exception.
                       .
                       .
                       .
                 end

However, as can be seen from the example, the code for raising the function_clause exception has been duplicated. The code duplication is no big deal in this simple example, but it would be in a function where the binary matching clause was followed by many other clauses. To avoid the code duplication, we must use letrec with the letrec_goto annotation:

   'foo'/1 =
       fun (_0) ->
           ( letrec
                 'label^0'/0 =
                     fun () ->
                           case _0 of
                             <_1> when 'true' ->
                                   %% Raise function_clause exception.
                                   .
                                   .
                                   .
                           end
             in  case _0 of
                   <#{#<Sz>(16,1,'integer',['unsigned'|['big']]),
                      #<_2>('all',1,'binary',['unsigned'|['big']])}#> when 'true' ->
                       case _2 of
                         <#{#<X>(Sz,1,'integer',['unsigned'|['big']])}#> when 'true' ->
                             X
                         <_3> when 'true' ->
                               apply 'label^0'/0()
                       end
                   <_4> when 'true' ->
                         apply 'label^0'/0()
                 end
             -| ['letrec_goto'] )

Core Erlang 的 `letrec` 有什么用？

What is Core Erlang's `letrec` for?

erlang