78 lines
2.7 KiB
Text
78 lines
2.7 KiB
Text
= ``zexp``s, store paths, and context
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:page-pagination:
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In Nix, strings carry around a "context", corresponding to a list of
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dependencies that string has. For example, the `outPath` of a derivation in Nix
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has a context that contains its derivation, plus the output name. When strings
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are concatenated, their contexts are merged together.
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In Zilch, a similar concept exists; encapsulated in so-called "zexpressions".
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A `zexpr` is a lazily evaluated thunk that, when evaluated, has access to other
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``zexpr``s and closely related objects. When a `zexpr` is unwrapped, it returns
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the resulting value, plus a list of derivations and outputs that are depended
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on.
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[source,scheme]
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----
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(define example (zexp foo))
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;; #<zexp val: foo>
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(zexp-unwrap example)
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;; #<zexp-evaluation val: foo; drvs: (); srcs: ()>
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----
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The primary way a zexp that contains context is made, is by using
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xref:generated:zilch.magic.adoc#store-path-for-drv[`store-path-for-drv`] and xref:generated:zilch.magic.adoc#store-path-for-ca-drv[`store-path-for-ca-drv`]. This returns a
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`<store-path>`, which can be unwrapped like a `zexp`.
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[source,scheme]
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----
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(define drv
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(store-path-for-drv
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"hello" "x86_64-linux"
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'("/bin/sh" "-c" "echo hi > $out") '()
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'("out"))
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;; (("out"
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;; . #<store path /nix/store/mjs27ix6ig2bkbi3s3sm470vrv4lf7ic-hello
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;; (/nix/store/76w21n1f03fs5kw8fnffphx7qrqffw6r-hello.drv!out)>))
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(zexp-unwrap (cdar drv))
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;; #<zexp-evaluation
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;; val: "/nix/store/mjs27ix6ig2bkbi3s3sm470vrv4lf7ic-hello";
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;; drvs: ((#<derivation …> "out"));
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;; srcs: ()>
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----
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A `zexp` behaves similarly to the `quasiquote` macro, in that it is possible to
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use `unquote` (or its reader syntax) to unquote values. Unquoting a `zexp`,
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however, requires a special mechanism, and is executed before `unquote`. This
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allows for full expressiveness when generating, for example, arguments or
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environments for derivations.
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[source,scheme]
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----
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(define complex
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(zexp
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(unquote
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(string-append
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"The path for the derivation's output is "
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(zexp-unquote (cdar #3)))))
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(zexp-unwrap complex)
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;; #<zexp-evaluation
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;; val: "The path for the derivation's output is /nix/store/mjs27ix6ig2bkbi3s3sm470vrv4lf7ic-hello";
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;; drvs: ((#<derivation …> "out"));
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;; srcs: ()>
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----
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A store path, after being created by `store-path-for-(ca-)drv`, is not
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guaranteed to exist in the store. To ensure it exists, xref:generated:zilch.magic.adoc#store-path-materialize[`store-path-materialize`]
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is used; this is transparently done when a `store-path` is unwrapped inside a
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`zexp`.
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== Reader syntax
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For convenience, `+#~foo+` is used as reader syntax for `(zexp foo)`, while
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`+#$foo+` and `+#$@foo+` are syntax for `(zexp-unwrap foo)` and `(zexp-unquote-splicing foo)`
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respectively.
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