lookup package:base-compat

lookup key assocs looks up a key in an association list. For the result to be Nothing, the list must be finite.

>>> lookup 2 []
Nothing

>>> lookup 2 [(1, "first")]
Nothing

>>> lookup 2 [(1, "first"), (2, "second"), (3, "third")]
Just "second"

Return the value of the environment variable var, or Nothing if there is no such value. For POSIX users, this is equivalent to getEnv.