Editing GHC/Error messages (section)

== "Parse error in pattern" ==

A "parse error in pattern" error occurs when you haven't matched on a data constructor in the way you meant to.

Example:

 merge [] bs rest = bs ++ rest
 merge as [] rest = as ++ rest
 merge a:as b:bs rest
     | a > b     = merge as (b:bs) (a:rest)
     | otherwise = merge (a:as) bs (b:rest)


 λ> :load
 [1 of 1] Compiling Main             ( merge.hs, interpreted )

 merge.hs:3:1: error: Parse error in pattern: merge
    |
  3 | merge a:as (b:bs) rest
    | ^^^^^^^
 Failed, no modules loaded.

This error occurs because we haven’t properly pattern matched on the data constructor in the parameter list for our last definition of the merge function. The type constructor here is a list, which means our options for data constructors are [] and a : [a].
The way we match on a list with multiple elements is by indicating it’s 1 argument and we construct the type with the data constructors. We construct a list of `a` elements is by writing it as `a` cons’ed onto a list of `as`, (a:as).

 merge [] bs rest = bs ++ rest
 merge as [] rest = as ++ rest
 merge (a:as) (b:bs) rest
     | a > b     = merge as (b:bs) (a:rest)
     | otherwise = merge (a:as) bs (b:rest)