RFC: add a command to help parsing arguments in scripts (#875)

the other day, i was writing a script and wanted to pass a `list<int>`
to it which is not possible because there is no such things as _types_
for externals 🤔

i ended up writing an "arg parsing" command to help in that task and
thought it could be useful to people 😇

in this MR, i add `std-rfc parse-arg` in the `script-parsing.nu` module
and add associated tests which all pass.

i invite the reader to have a look at the docstring of `parse-arg` which
should contain a full example explaining the usage of this new command
😉

---------

Co-authored-by: Darren Schroeder <343840+fdncred@users.noreply.github.com>

stdlib-candidate/std-rfc/mod.nu

@@ -6,3 +6,4 @@ export module math/
 export use bulk-rename.nu *
 export use set-env.nu *
 export use bench.nu
+export use script-parsing.nu [ parse-arg ]

stdlib-candidate/std-rfc/script-parsing.nu

@@ -0,0 +1,61 @@
+# helps parsing CLI arguments for Nushell scripts
+#
+# the following Nushell script does not make sense to be used as an external
+# command because there is no such thing as a `list<string>` in Bash for
+# instance.
+# ```nushell
+# def main [x: list<int>] {
+#     print $x
+# }
+# ```
+#
+# one needs to write something less strict at parse-time and thus looses type
+# information...
+# ```nushell
+# def main [
+#     x: string, # list<int>
+# ] {
+#     print $x
+# }
+# ```
+#
+# it's possible to write a much stronger script whith `parse-arg`
+# ```nushell
+# def main [
+#     x: string, # list<int>
+# ] {
+#     let x = $x | parse-arg (metadata $x).span "list<int>" # the script would crash if either
+#                                                           # `$x: string` is not valid NUON or if
+#                                                           # the resulting value is not a `list<int>`
+#     print $x                                              # here, `$x` is a `list<int>` as intended
+# }
+# ```
+export def parse-arg [
+    span: record<start: int, end: int>, # the span of the input variable
+    expected_type: string, # the expected type for the input variable
+]: [ string -> any ] {
+    let val = try {
+        $in | from nuon
+    } catch {
+        error make {
+            msg: $"(ansi red_bold)invalid NUON(ansi reset)",
+            label: {
+                text: "invalid NUON",
+                span: $span,
+            },
+        }
+    }
+
+    if ($val | describe) != $expected_type {
+        error make {
+            msg: $"(ansi red_bold)bad type(ansi reset)",
+            label: {
+                text: $"type: ($val | describe)",
+                span: $span,
+            },
+            help: $"expected ($expected_type)",
+        }
+    }
+
+    $val
+}

stdlib-candidate/tests/mod.nu

@@ -3,4 +3,5 @@ export module record.nu
 export module str_xpend.nu
 export module math.nu
 export module bench.nu
+export module script-parsing.nu
 export module str_dedent.nu

stdlib-candidate/tests/script-parsing.nu

@@ -0,0 +1,37 @@
+use std assert
+use ../std-rfc parse-arg
+
+const SPAN = { start: 0, end: 0 }
+
+export def "test parse-arg ok" [] {
+    const TEST_CASES = [
+        [ input, type, expected ];
+
+        [ "123", "int", 123 ],
+        [ "[1, 2, 3]", "list<int>", [1, 2, 3] ],
+        [ "'spam'", "string", "spam" ],
+        [
+            "{ a: 1, b: 'egg', c: false }",
+            "record<a: int, b: string, c: bool>",
+            { a: 1, b: 'egg', c: false },
+        ],
+    ]
+
+    for t in $TEST_CASES {
+        assert equal ($t.input | parse-arg $SPAN $t.type) $t.expected
+    }
+}
+
+export def "test parse-arg err" [] {
+    const TEST_CASES = [
+        [ input, type ];
+
+        [ "{ invalid NUON", "" ],
+        [ "[1, 2, 3]", "string" ],
+    ]
+
+    for t in $TEST_CASES {
+        let msg = $"test case: input: '($t.input)', type: ($t.type)"
+        assert error { $t.input | parse-arg $SPAN $t.type } $msg
+    }
+}