placement of curly brace blocks in DCG rules

[mvolkmann]

I haven't found any information on where curly brace blocks should be placed inside DCG rules. For example, in the following code, the curly brace block sets a variable that is used before the block and another that is used after the block. Is the order irrelevant or are there cases where it matters?

:- use_module(library(dcgs)).

sample(S) -->
  seq(First),
  {
    First = "one",
    Second = "two",
    S = "three"
  },
  seq(Second).

I can test this by entering phrase(sample(S), X). which yields S = "three", X = "onetwo"

[UWN]

I haven't found any information on where curly brace blocks should be placed inside DCG rules.

See my previous comment on failure slices. It answered your question about the order.

[mvolkmann]

Thanks! What I derived from that is ...
Curly brace blocks that set variables used in the DCG body should appear before the body goal that uses them.

[UWN]

Curly brace blocks that set variables used in the DCG body should appear before the body goal that uses them.

Provided these variables actually influence termination. Use as such is often irrelevant.

[mvolkmann]

In my simple use case above it does seem to affect termination. Entering phrase(sample(S), X). did not terminate as the code is written, but moving the curly brace block (grammar goal) to before seq(First) fixed it so it terminates.