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Tide doesn’t have “statements” in the traditional sense. Instead, you combine:
  • expressions (if ... then ... else ...)
  • comprehensions (list-building loops)
  • relation clauses + guards (when)
That’s enough to write surprisingly expressive programs.

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Conditionals: if / then / else

if is an expression, so it always produces a value. 
query if 5 > 3 then "yes" else "no"
Because it’s an expression, you can nest it: 
query if 0 > 1 then 10 else if 0 == 0 then 20 else 30

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Guards: when

Relation clauses can be guarded with when. 
relation abs where
  abs(x) = x     when x >= 0
  abs(x) = -x    when x < 0

query abs(-5)
query abs(5)
Guards are Tide’s “control flow” inside relations: they decide which clause applies.

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Looping by building: comprehensions

Instead of a for statement, Tide has list comprehensions: 
query [x * x for x in 1..5]
You can add filtering with when: 
query [x for x in 1..20 when x % 2 == 0]

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The vibe

Tide tries to keep the surface area small:
  • expressions for branching
  • comprehensions for iteration
  • relations for “rules”
Once you get used to it, the language starts to feel like a tiny logic-flavored calculator you can teach new tricks.