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Add more documentation to declaring conditionals. #141

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Add more documentation to declaring conditionals. #141

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43 changes: 43 additions & 0 deletions README.md
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Original file line number Diff line number Diff line change
Expand Up @@ -134,6 +134,49 @@ becomes
extra-lib-dirs:
lib/darwin

Multiple conditionals must fall under one `when`, for example:

when:
- condition: os(darwin)
extra-lib-dirs: lib/darwin
- condition: os(linux)
extra-lib-dirs: /usr/lib/x64

becomes

if os(darwin)
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Minor note generally, but cabal library converts Darwin to osx

extra-lib-dirs:
lib/darwin
if os(linux)
extra-lib-dirs:
/usr/lib/x64

If conditionals are declared under different `when` clauses only the last one
will make it to the `.cabal` file:
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@sol sol Jan 8, 2017 •
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I think it may not necessarily be the last one. This is again resulting from how YAML works. This is an object (think of it as a hash map), duplicate keys are not allowed.

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You're right that this behaviour is on the YAML side of things (and is actually wrong according to the spec: http://yaml.org/spec/1.1/#id932806), but I find it very useful that it behaves as it does, and would actually propose to add tests for this behaviour and file bug reports to https://hackage.haskell.org/package/yaml if the behaviour ever changes. Here are two examples why:

Aliases with overriding:

executables:
    strava-gear-cmdline:
        &default_executable
        main: strava-gear-cmdline.hs
        dependencies: [strava-gear]
        ghc-options:
            - -threaded

    strava-gear-web:
        <<: *default_executable
        main: strava-gear-web.hs

Predefined aliases:

executables:
    &default_executable
    dependencies: [strava-gear]
    ghc-options:
        - -threaded

executables:
    strava-gear-cmdline:
        <<: *default_executable
        main: strava-gear-cmdline.hs

    strava-gear-web:
        <<: *default_executable
        main: strava-gear-web.hs

The second example is a massive hack, but I'm not aware of any other way to do this given that hpack insists on printing warnings for unused fields even if I prefix them with underscores.

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@liskin Hey, interesting! How about we don't emit warnings for unknown top-level field that starts with an underscore? That way you can use fields that start with underscores for building kind of a hpack library.

I would certainly accept a patch. So if you think thats the way to go, then feel free to open a PR!

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Here you go: #146 :-)


when:
- condition: os(darwin)
extra-lib-dirs: lib/darwin
when:
- condition: os(linux)
extra-lib-dirs: /usr/lib/x64

becomes:

if os(linux)
extra-lib-dirs:
/usr/lib/x64

Unlike with `.cabal` files compound predicates need to be in parens, for example:

when:
- condition: (!os(darwin) && !os(windows))
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@sol sol Jan 8, 2017 •
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I believe the issue here is not that you have a "compound predicate" here. It's merely the !, which in YAML must be quoted when at the beginning of a string, say:

condition: "!os(darwin) && !os(windows)"

ghc-options: ...

If they are not the YAML parser will throw this mysterious error:

./package.yaml:41:31: did not find expected alphabetic or numeric character while scanning an anchor

Conditionals with an else branch:

- Must have a `condition` field
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