Haskell: Constraining function pattern variables with |

Question

While I was browsing Persistent source code, I came across this function below in file Quasi.hs (I referred to a tag with the relevant code equal to the one in the present state in master branch, since the tag's code is more unlikely to change). In the line takeConstraint ps tableName defs (n:rest) | not (T.null n) && isUpper (T.head n) = takeConstraint' there is this pipe (|) character following the argument pattern. Is the expression between | and = like a constraint to arguments in the pattern? So do I interpret this | as the same symbol in Math, i.e. "such that"?

takeConstraint :: PersistSettings
          -> Text
          -> [FieldDef]
          -> [Text]
          -> (Maybe FieldDef, Maybe CompositeDef, Maybe UniqueDef, Maybe UnboundForeignDef)
takeConstraint ps tableName defs (n:rest) | not (T.null n) && isUpper (T.head n) = takeConstraint' --- <<<<< This line
    where
      takeConstraint' 
            | n == "Unique"  = (Nothing, Nothing, Just $ takeUniq ps tableName defs rest, Nothing)
            | n == "Foreign" = (Nothing, Nothing, Nothing, Just $ takeForeign ps tableName defs rest)
            | n == "Primary" = (Nothing, Just $ takeComposite defs rest, Nothing, Nothing)
            | n == "Id"      = (Just $ takeId ps tableName (n:rest), Nothing, Nothing, Nothing)
            | otherwise      = (Nothing, Nothing, Just $ takeUniq ps "" defs (n:rest), Nothing) -- retain compatibility with original unique constraint
takeConstraint _ _ _ _ = (Nothing, Nothing, Nothing, Nothing)

leftaroundabout · Accepted Answer · 2017-03-27 16:25:46Z

6

Yes, it means precisely “such that”. These are guards, very common in Haskell (generally preferred to the equivalent if then else expression).

f x
 | x > 2      = a
 | x < -4     = b
 | otherwise  = x

is equivalent to

f x = if x > 2 then a
               else if x < -4 then b
                              else x

IMO that specific example would actually have better been written with neither if nor guards, but

    takeConstraint' = case n of
        "Unique"  -> (Nothing, Nothing, Just $ takeUniq ps tableName defs rest, Nothing)
        "Foreign" -> (Nothing, Nothing, Nothing, Just $ takeForeign ps tableName defs rest)
        "Primary" -> (Nothing, Just $ takeComposite defs rest, Nothing, Nothing)
        "Id"      -> (Just $ takeId ps tableName (n:rest), Nothing, Nothing, Nothing)
        _         -> (Nothing, Nothing, Just $ takeUniq ps "" defs (n:rest), Nothing)

edited Mar 27, 2017 at 16:25

answered Mar 27, 2017 at 16:13

leftaroundabout

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2 Comments

chi Over a year ago

The case looks better for me as well.

Rafael Over a year ago

I agree, cramming a whole function block between after the guard was a bit misleading, I thought it was a completely novel feature from Haskell.

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