Built-ins

This page covers the top-level built-ins that are always in scope even though they are not module members.

Values and functions

Name	Signature	Description
Env	Environment	Host-injected execution metadata for CLI file runs, stdin runs, and the REPL.
print	string -> unit	Writes a line of text to standard output.
ignore	'a -> unit	Discards a value when only the side effect of an expression matters.

type Environment

type Environment =
  { ScriptName: string option
    Arguments: string list }

The host exposes Env with this shape:

• Env.ScriptName is Some "path/to/script.fss" for file execution.
• Env.ScriptName is None for stdin execution and the REPL.
• Env.Arguments contains arguments passed after --.

match Env.Arguments with
| name :: _ -> print $"hello {name}"
| [] -> print "hello"

type FsKind

type FsKind =
  | File of string
  | Directory of string
  | Missing

Fs.kind returns FsKind, which lets scripts branch on filesystem state with pattern matching:

match Fs.kind "samples" with
| Directory path -> print $"dir: {path}"
| File path -> print $"file: {path}"
| Missing -> print "missing"

print : string -> unit

Use print for script output and quick debugging.

print "hello"

ignore : 'a -> unit

Use ignore when an expression returns a value you intentionally do not need.

String.indexOf "x" "text" |> ignore