MLton 20210117

# MLBasisAnnotationExamples

Here are some example uses of MLBasisAnnotations.

## Eliminate spurious warnings in automatically generated code

Programs that automatically generate source code can often produce nonexhaustive patterns, relying on invariants of the generated code to ensure that the pattern matchings never fail. A programmer may wish to elide the nonexhaustive warnings from this code, in order that legitimate warnings are not missed in a flurry of false positives. To do so, the programmer simply annotates the generated code with the nonexhaustiveBind ignore and nonexhaustiveMatch ignore annotations:

    local
      $(GEN_ROOT)/gen-lib.mlb

      ann
        "nonexhaustiveBind ignore"
        "nonexhaustiveMatch ignore"
      in
        foo.gen.sml
      end
    in
      signature FOO
      structure Foo
    end

## Deliver a library

Standard ML libraries can be delivered via .mlb files. Authors of such libraries should strive to be mindful of the ways in which programmers may choose to compile their programs. For example, although the defaults for sequenceNonUnit and warnUnused are ignore and false, periodically compiling with these annotations defaulted to warn and true can help uncover likely bugs. However, a programmer is unlikely to be interested in unused modules from an imported library, and the behavior of sequenceNonUnit error may be incompatible with some libraries. Hence, a library author may choose to deliver a library as follows:

    ann
      "nonexhaustiveBind warn" "nonexhaustiveMatch warn"
      "redundantBind warn" "redundantMatch warn"
      "sequenceNonUnit warn"
      "warnUnused true" "forceUsed"
    in
      local
        file1.sml
        ...
        filen.sml
      in
        functor F1
        ...
        signature S1
        ...
        structure SN
        ...
      end
    end

The annotations nonexhaustiveBind warn, redundantBind warn, nonexhaustiveMatch warn, redundantMatch warn, and sequenceNonUnit warn have the obvious effect on elaboration. The annotations warnUnused true and forceUsed work in conjunction — warning on any identifiers that do not contribute to the exported modules, and preventing warnings on exported modules that are not used in the remainder of the program. Many of the available libraries are delivered with these annotations.