#lang scribble/manual

@title{fun}

@verbatim|{
fun <id>(<id> {, <id>}*): <test-expr> <newline>
fun <id>(<id> {, <id>}*): <newline> <local-defs>* <expr>
}|

This piece of syntax defines a function. For example,
@tt[]{fun plus1(x): x + 1} defines a function called @tt[]{plus1},
which consumes one variable (@tt{x}), and evaluates to
@tt{x + 1}. The second form allows local definitions, so,
for example,

@verbatim|{
fun outer(x):
  fun inner(y): y
  inner(x)
}|

defines two functions, one called @tt{outer}, and one called
@tt{inner}. The function @tt{inner} is only available in the
scope of @tt{outer}.

Functions are required to consume at least one argument. Furthermore,
functions are first-order in BSL -- after defining a function called
@tt[]{outer}, the expression @tt[]{outer} would result in an error.

Function application looks like it does in algebra:

@verbatim|{
fun plus1(x): x + 1

> plus1(1)
2
}|

@tt[]{fun} follows the same-line/greater-column rule. So, typing

@verbatim|{
fun plus1(x):
x + 1
}|

results in an indentation error.

However, if local definitions are present, all such definitions must
follow the same-line/same-column rule. So,

@verbatim|{
fun add3(x):
  def one: 1
  def two: 2
  x + one + two
}|

is indented correctly, but

@verbatim|{
fun add3(x):
  def one: 1
def two: 2
  x + one + two
}|

is not.