This post is inspired by a delightful insight from Luke Palmer. I’ll begin with some motivation, and then propose a puzzle.
Consider the following definition of our familiar conditional:
ifThenElse :: Bool → a → a → a ifThenElse True t f = t ifThenElse False t f = f
In a strict language, where there are only two boolean values, these two clauses have a straightforward reading.
(The reading is less straightforward when patterns overlap, as mentioned in Lazier function definitions by merging partial values.)
In a non-strict language like Haskell, there are three distinct boolean values, not two.
Bool also has a value ⊥, pronounced “bottom” for being at the bottom of the information ordering.
For an illustration and explanation of information ordering, see Merging partial values.
Note: In Haskell code, ⊥ is sometimes denoted by “
undefined“, which can be confusing, because the meaning is defined precisely.
There are many other ways to denote ⊥ in Haskell, and it is impossible to determine whether or not an arbitrary Haskell expression denotes ⊥.
I’ll generally use “⊥” in place of “
undefined” in this post, as well as for the corresponding semantic value.
The two-clause definition above only addresses two of the three possible boolean values explicitly.
What, if anything, does it say indirectly about the meaning of an application like “
ifThenElse ⊥ 3 5“?
The Haskell language standard gives an operational answer to this question. Clauses are examined, using pattern matching to select a clause and instantiate that clause’s variables. In case more than one clause matches, the earlier one is chosen.
Pattern matching has three possible outcomes:
- A single substitution, providing variable bindings that specialize the patterns in a clause’s left-hand side (LHS) to coincide with the actual call. The matching uses semantic, not syntactic, equality and can require forcing evaluation of previously unevaluated thunks (delayed computations).
- Proof of the nonexistence of such a substitution.
- Neither conclusion, due to an error or nontermination during evaluation.
In this example, the effort to match
True against ⊥ leads to the third outcome.
For Haskell as currently defined, the result of the application in such a case is then defined to be ⊥ also.
Which is to say that
ifThenElse is strict (in its first argument).
So strictness is the Haskell answer, but is it really the answer we want? Are there alternatives that might better fit the spirit of non-strict functional programming?