add std.contract.Sequence (#1452)

* add std.contract.Sequence

* nickel lists have `,`

Co-authored-by: Viktor Kleen <viktor.kleen@tweag.io>

* eta reduction

Co-authored-by: Yann Hamdaoui <yann.hamdaoui@tweag.io>

* example where it's actually useful

* capitalisation

---------

Co-authored-by: Viktor Kleen <viktor.kleen@tweag.io>
Co-authored-by: Yann Hamdaoui <yann.hamdaoui@tweag.io>

core/stdlib/std.ncl

@@ -934,6 +934,41 @@
         "%
       = fun pred label value => if pred value then value else %blame% label,
 
+    Sequence
+      | doc m%"
+          Apply multiple contracts from left to right.
+
+          Type: `Array Contract -> Contract`
+          (for technical reasons, this function isn't actually statically typed)
+
+          # Examples
+
+          ```nickel
+          x | std.contract.Sequence [ Foo, Bar ]
+          ```
+
+          is equivalent to
+
+          ```nickel
+          (x | Foo) | Bar
+          ```
+
+          This is useful in positions where one cannot apply multiple contracts,
+          such as an argument to another contract:
+
+          ```nickel
+          x | Array (std.contract.Sequence [ Foo, Bar ])
+          ```
+
+          Or stored in a variable:
+
+          ```nickel
+          let C = std.contract.Sequence [ Foo, Bar ] in
+          x | C
+          ```
+        "%
+      = fun contracts label value =>
+        std.array.fold_right (fun contract => std.contract.apply contract label) value contracts,
     label
       | doc m%"
           The label submodule provides functions that manipulate the label

core/tests/integration/fail/contracts/sequence.ncl

@@ -0,0 +1,6 @@
+# test.type = 'error'
+#
+# [test.metadata]
+# error = 'EvalError::BlameError'
+
+3 | std.contract.Sequence [ Number, std.contract.from_predicate (fun x => x < 5), std.contract.from_predicate (fun x => x > 5) ]

core/tests/integration/pass/contracts/contracts.ncl

@@ -166,5 +166,11 @@ let {check, Assert, ..} = import "../lib/assert.ncl" in
         = fun r => %record_insert% "g" r g in
   let res = f { z = 3 }
   in true,
+
+  # std.contract.Sequence
+  let three = 3 | std.contract.Sequence [ Number, std.contract.from_predicate (fun x => x < 5) ] in
+  # preserves order
+  let tag = "some_tag" | std.contract.Sequence [ std.enum.TagOrString, [| 'some_tag |] ]
+  in true
 ]
 |> check