'# Syntax of if expressions

'The basic syntax of `if` in Dex is
```
if <condition> then <consequent> [else <alternate>]
```

'It can be a bit confusing, though, because of all the tokens it may make sense to indent.

'The main rules are:
- The `else` clause is optional (regardless of indentation)
- The `then` and `else` keywords can be inline with the preceding
  code, or indented relative to the `if`.
- The code for each arm of the `if` can be either an inline expression
  or start a new indentation level (relative to its keyword if that is
  indented, or relative to the whole `if` otherwise).

'This produces four combinations for one-armed `if`, all of which are legal:

:p
  yield_accum (AddMonoid Float) \ref.
    if True then ref += 3.
    if True then
      ref += 1.
      ref += 2.
    if True
      then ref += 3.
    if False
      then
        ref += 1.
        ref += 2.
> 9.

'However, not every one of the 16 concievable combinations makes sense for two-armed `if`.
To wit:
- If the consequent is indented, it makes no sense to have the `else`
  inline (eliminating 4 combinations).
- If `then` is inline, there can be no indented `else` either, because
  there is no readable level at which to indent it (elimintaing 2 more
  combinations).

'The following contrived code block shows all the acceptable configurations:

:p
  yield_accum (AddMonoid Float) \ref.
    -- Two-armed `if` with `then` and the consequent both inline.
    x = if False then 1. else 3.
    if False then ref += 100. else
      ref += 1.
      ref += 2.
    if False then ref += 200.
      else ref += x
    if False then ref += 300.
      else
        ref += 1.
        ref += 2.

    -- Two-armed `if` with `then` indented but the consequent inline.
    y = if False
          then 1. else 3.
    if False
      then ref += 100. else
        ref += 1.
        ref += 2.
    if False
      then ref += 200.
      else ref += y
    if False
      then ref += 300.
      else
        ref += 1.
        ref += 2.

    -- Two-armed `if` with `then` and the consequent both indented.
    if False
      then
        ref += 100.
        ref += 200.
      else ref += 3.
    if False
      then
        ref += 100.
        ref += 200.
      else
        ref += 2.
        ref += 4.
> 27.

'And here are expample configurations that don't work, showing the resulting parse errors.

'Inline `else` is not allowed after indented consequent, whether the
`then` keyword is indented or not:

if True
  then
    x = 6
    x else 5

> Parse error:97:12:
>    |
> 97 |     x else 5
>    |            ^
> Same-line `else` may not follow indented consequent; put the `else` on the next line.

if True then
  x = 6
  x else 5

> Parse error:107:10:
>     |
> 107 |   x else 5
>     |          ^
> No `else` may follow same-line `then` and indented consequent; indent and align both `then` and `else`, or write the whole `if` on one line.

'Indented `else` is not allowed after inline `then` and indented
consequent either, because there is no indentation level for it to match.

:p
 if True then
   x = 6
   x
  else 5

> Parse error:122:8:
>     |
> 122 |   else 5
>     |        ^
> No `else` may follow same-line `then` and indented consequent; indent and align both `then` and `else`, or write the whole `if` on one line.